Flex 布局

Flexbox 是 CSS 的一维弹性布局模型，通过容器与项目的双轴协作，让空间分配与对齐从"手动计算"变为"声明式驱动"。

核心概念

Flex 容器与项目

Flex 布局的核心模型是 容器-项目 二层结构。当一个元素的 display 设为 flex 或 inline-flex 时，它成为 Flex 容器，其所有直接子元素自动成为 Flex 项目。

css

/* 块级 Flex 容器：独占一行 */
.container {
  display: flex;
}

/* 行内 Flex 容器：宽度由内容撑开，可与其他行内元素并排 */
.inline-container {
  display: inline-flex;
}

理解容器与项目的关系需要牢记以下规则：

规则	说明
仅限直接子元素	只有容器的直接子元素才是 Flex 项目，孙级元素不受影响
匿名 Flex 项目	容器内的裸文本节点会被自动包装为匿名 Flex 项目
绝对定位例外	设置了 `position: absolute` 或 `fixed` 的子元素脱离 Flex 布局，不参与伸缩与对齐
嵌套容器	Flex 项目本身也可以是 Flex 容器，形成嵌套布局
float 失效	Flex 项目上的 `float` 和 `clear` 无效
vertical-align 失效	Flex 项目上的 `vertical-align` 无效
外边距不折叠	Flex 项目之间的垂直外边距不会折叠（不同于普通流中的块级元素）

主轴与交叉轴

Flex 布局基于 双轴模型 运作：主轴（Main Axis）决定项目的排列方向，交叉轴（Cross Axis）与主轴垂直，决定项目的对齐方式。所有对齐属性都围绕这两条轴定义，理解轴向是掌握 Flex 布局的前提。

轴向术语：

术语	含义	相关属性
main start / main end	主轴起点 / 终点	`justify-content`、`flex-grow`
cross start / cross end	交叉轴起点 / 终点	`align-items`、`align-content`
main size	主轴方向尺寸	`flex-basis`、`width`（row 时）
cross size	交叉轴方向尺寸	`height`（row 时）

主轴方向由 flex-direction 决定，交叉轴始终与主轴垂直。不同 flex-direction 值下两轴的方向关系如下：

图表渲染中…

关键认知：justify-content 永远作用于主轴，align-items 永远作用于交叉轴——它们不关心轴是水平还是垂直。当 flex-direction: column 时，justify-content 控制的是垂直方向对齐，align-items 控制的是水平方向对齐。

容器属性详解

Flex 容器属性控制项目的排列方向、换行行为、对齐方式和间距。以下属性均设置在 Flex 容器上。

flex-direction

flex-direction 决定主轴方向，即项目的排列方向。它是 Flex 布局中最基础的属性，直接影响所有对齐属性的作用方向。

值	主轴方向	交叉轴方向	说明
`row`	水平 →	垂直 ↓	默认值，从左到右
`row-reverse`	水平 ←	垂直 ↓	从右到左
`column`	垂直 ↓	水平 →	从上到下
`column-reverse`	垂直 ↑	水平 →	从下到上

html

<div class="container row">A B C</div>
<div class="container col">A B C</div>

css

.container {
  display: flex;
  gap: 10px;
  padding: 10px;
  border: 2px solid #1976d2;
  margin-bottom: 10px;
}

/* 水平排列（默认） */
.row {
  flex-direction: row;
}

/* 垂直排列 */
.col {
  flex-direction: column;
}

注意：row-reverse 和 column-reverse 仅改变视觉排列方向，不改变 DOM 顺序。屏幕阅读器等辅助技术仍按 DOM 顺序读取内容。

flex-wrap

flex-wrap 控制项目是否换行。默认情况下，Flex 项目会尽量挤在一行中，即使被压缩到小于内容宽度。

值	行为
`nowrap`	默认值，不换行，项目被压缩以适应单行
`wrap`	换行，第一行在上方
`wrap-reverse`	换行，第一行在下方

html

<div class="container">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
  <div class="item">4</div>
  <div class="item">5</div>
</div>

css

.container {
  display: flex;
  flex-wrap: wrap; /* 允许换行 */
  gap: 10px;
}

.item {
  /* 每个项目最小 200px，空间不足时自动换行 */
  flex: 1 1 200px;
  min-width: 0;
  height: 80px;
  background: #e3f2fd;
  border: 1px solid #1976d2;
}

要点：nowrap 下项目会被压缩（受 flex-shrink 控制）；wrap 下项目不会被压缩，空间不足时自动折到下一行。需要响应式布局时，flex-wrap: wrap 配合 flex-basis 是常用模式。

flex-wrap 交互演示（MDN）

设置 flex 项目是否换行（nowrap/wrap/wrap-reverse）。

html

<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>flex-wrap 属性演示 - MDN 示例</title>
    <meta name="description" content="演示flex（wrap 属性演示）效果。" />
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }
      body {
        font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
        min-height: 100vh;
        padding: 0;
      }
      .demo-layout {
        display: flex;
        height: 100vh;
        gap: 0;
      }
      .snippet-panel {
        width: 320px;
        flex-shrink: 0;
        padding: 16px;
        display: flex;
        flex-direction: column;
        gap: 8px;
        overflow-y: auto;
        border-right: 1px solid #ddd;
        background: #fafafa;
      }
      .snippet-btn {
        padding: 10px 14px;
        border: 1px solid #ccc;
        border-radius: 6px;
        background: #fff;
        font-family: "JetBrains Mono", "Fira Code", monospace;
        font-size: 13px;
        color: #333;
        cursor: pointer;
        text-align: left;
        transition: all 0.2s;
        line-height: 1.4;
      }
      .snippet-btn:hover {
        border-color: #8083ff;
        background: #f0f0ff;
      }
      .snippet-btn.active {
        border-color: #8083ff;
        background: #e8e8ff;
        color: #571bc1;
        font-weight: 600;
      }
      .preview-panel {
        flex: 1;
        padding: 16px;
        display: flex;
        align-items: center;
        justify-content: center;
        background: #fff;
        overflow: auto;
      }
      .preview-panel > section,
      .preview-panel > div:not(.snippet-panel):not(.demo-layout) {
        flex: 1;
        width: 100%;
        min-height: 0;
        height: 100%;
        display: flex;
        align-items: center;
        justify-content: center;
      }
      #example-element {
        border: 1px solid #c5c5c5;
        width: 80%;
        display: flex;
      }

      #example-element > div {
        background-color: rgba(0, 0, 255, 0.2);
        border: 3px solid blue;
        width: 60px;
        margin: 10px;
      }
    </style>
  </head>
  <body>
    <div class="demo-layout">
      <div class="snippet-panel">
        <button class="snippet-btn active" data-index="0">flex-wrap: nowrap;</button>
        <button class="snippet-btn" data-index="1">flex-wrap: wrap;</button>
        <button class="snippet-btn" data-index="2">flex-wrap: wrap-reverse;</button>
      </div>
      <div class="preview-panel">
        <section class="default-example" id="default-example">
          <div class="transition-all" id="example-element">
            <div>Item One</div>
            <div>Item Two</div>
            <div>Item Three</div>
            <div>Item Four</div>
            <div>Item Five</div>
            <div>Item Six</div>
          </div>
        </section>
      </div>
    </div>
    <script>
      const snippets = [
        `#example-element {
  flex-wrap: nowrap;
}`,
        `#example-element {
  flex-wrap: wrap;
}`,
        `#example-element {
  flex-wrap: wrap-reverse;
}`,
      ];

      let styleEl = document.createElement("style");
      document.head.appendChild(styleEl);

      function applySnippet(index) {
        styleEl.textContent = snippets[index];
        document.querySelectorAll(".snippet-btn").forEach((btn, i) => {
          btn.classList.toggle("active", i === index);
        });
      }

      document.querySelectorAll(".snippet-btn").forEach((btn) => {
        btn.addEventListener("click", () => applySnippet(parseInt(btn.dataset.index)));
      });

      applySnippet(0);
    </script>
  </body>
</html>

flex-flow

flex-flow 是 flex-direction 和 flex-wrap 的简写属性，默认值为 row nowrap。

css

.container {
  display: flex;

  /* 完整写法：方向 + 换行 */
  flex-flow: row wrap;

  /* 等同于分开写 */
  /* flex-direction: row; */
  /* flex-wrap: wrap; */
}

flex-flow 的两个值可以互换顺序，浏览器会自动识别。实际开发中，如果方向为默认的 row，通常只写 flex-wrap 即可。

justify-content

justify-content 控制项目在主轴上的对齐方式，决定剩余空间如何分配。这是使用频率最高的 Flex 属性之一。

值	对齐方式	首项目前间距	项目间间距	尾项目后间距
`flex-start`	起点对齐	0	0	全部剩余
`flex-end`	终点对齐	全部剩余	0	0
`center`	居中对齐	剩余/2	0	剩余/2
`space-between`	两端对齐	0	剩余/(n-1)	0
`space-around`	等距环绕	剩余/2n	剩余/n	剩余/2n
`space-evenly`	均匀分布	剩余/(n+1)	剩余/(n+1)	剩余/(n+1)

n 为项目数量。当只有一个项目时，space-between 等同于 flex-start。

html

<div class="container">
  <div class="item">A</div>
  <div class="item">B</div>
  <div class="item">C</div>
</div>

css

.container {
  display: flex;
  justify-content: space-between; /* 两端对齐，项目间间距相等 */
  padding: 10px;
  border: 2px solid #1976d2;
}

.item {
  width: 80px;
  height: 40px;
  background: #e3f2fd;
  border: 1px solid #1976d2;
  display: flex;
  justify-content: center;
  align-items: center;
}

注意：justify-content 仅在有剩余空间时生效。如果项目设置了 flex-grow 占满了主轴，或项目总宽度已超出容器，则对齐效果不可见。

justify-content 交互演示（MDN）

设置 flex/grid 容器主轴方向项目的对齐与分布方式。

html

<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>justify-content 属性演示 - MDN 示例</title>
    <meta name="description" content="演示justify（content 属性演示）效果。" />
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }
      body {
        font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
        min-height: 100vh;
        padding: 0;
      }
      .demo-layout {
        display: flex;
        height: 100vh;
        gap: 0;
      }
      .snippet-panel {
        width: 320px;
        flex-shrink: 0;
        padding: 16px;
        display: flex;
        flex-direction: column;
        gap: 8px;
        overflow-y: auto;
        border-right: 1px solid #ddd;
        background: #fafafa;
      }
      .snippet-btn {
        padding: 10px 14px;
        border: 1px solid #ccc;
        border-radius: 6px;
        background: #fff;
        font-family: "JetBrains Mono", "Fira Code", monospace;
        font-size: 13px;
        color: #333;
        cursor: pointer;
        text-align: left;
        transition: all 0.2s;
        line-height: 1.4;
      }
      .snippet-btn:hover {
        border-color: #8083ff;
        background: #f0f0ff;
      }
      .snippet-btn.active {
        border-color: #8083ff;
        background: #e8e8ff;
        color: #571bc1;
        font-weight: 600;
      }
      .preview-panel {
        flex: 1;
        padding: 16px;
        display: flex;
        align-items: center;
        justify-content: center;
        background: #fff;
        overflow: auto;
      }
      .preview-panel > section,
      .preview-panel > div:not(.snippet-panel):not(.demo-layout) {
        flex: 1;
        width: 100%;
        min-height: 0;
        height: 100%;
        display: flex;
        align-items: center;
        justify-content: center;
      }
      #example-element {
        border: 1px solid #c5c5c5;
        width: 220px;
        display: grid;
        grid-template-columns: 60px 60px;
        grid-auto-rows: 40px;
        row-gap: 10px;
      }

      #example-element > div {
        background-color: rgba(0, 0, 255, 0.2);
        border: 3px solid blue;
      }
    </style>
  </head>
  <body>
    <div class="demo-layout">
      <div class="snippet-panel">
        <button class="snippet-btn active" data-index="0">justify-content: start;</button>
        <button class="snippet-btn" data-index="1">justify-content: center;</button>
        <button class="snippet-btn" data-index="2">justify-content: space-between;</button>
        <button class="snippet-btn" data-index="3">justify-content: space-around;</button>
        <button class="snippet-btn" data-index="4">justify-content: space-evenly;</button>
      </div>
      <div class="preview-panel">
        <section class="default-example" id="default-example">
          <div class="example-container">
            <div class="transition-all" id="example-element">
              <div>One</div>
              <div>Two</div>
              <div>Three</div>
            </div>
          </div>
        </section>
      </div>
    </div>
    <script>
      const snippets = [
        `#example-element {
  justify-content: start;
}`,
        `#example-element {
  justify-content: center;
}`,
        `#example-element {
  justify-content: space-between;
}`,
        `#example-element {
  justify-content: space-around;
}`,
        `#example-element {
  justify-content: space-evenly;
}`,
      ];

      let styleEl = document.createElement("style");
      document.head.appendChild(styleEl);

      function applySnippet(index) {
        styleEl.textContent = snippets[index];
        document.querySelectorAll(".snippet-btn").forEach((btn, i) => {
          btn.classList.toggle("active", i === index);
        });
      }

      document.querySelectorAll(".snippet-btn").forEach((btn) => {
        btn.addEventListener("click", () => applySnippet(parseInt(btn.dataset.index)));
      });

      applySnippet(0);
    </script>
  </body>
</html>

align-items

align-items 控制项目在交叉轴上的对齐方式。当 flex-direction: row 时，它控制垂直对齐；当 flex-direction: column 时，它控制水平对齐。

值	行为
`stretch`	默认值，拉伸填满容器交叉轴方向（需项目未设置交叉轴尺寸）
`flex-start`	交叉轴起点对齐
`flex-end`	交叉轴终点对齐
`center`	交叉轴居中对齐
`baseline`	项目的文本基线对齐

html

<div class="container">
  <div class="item tall">A</div>
  <div class="item">B</div>
  <div class="item small">C</div>
</div>

css

.container {
  display: flex;
  align-items: center; /* 交叉轴居中 */
  height: 200px;
  border: 2px solid #1976d2;
}

.item {
  width: 80px;
  background: #e3f2fd;
  border: 1px solid #1976d2;
  display: flex;
  justify-content: center;
  align-items: center;
}

.tall { height: 120px; }
.small { height: 40px; }

stretch 的陷阱：如果项目显式设置了交叉轴方向的尺寸（如 height），stretch 不会拉伸它。只有未设置尺寸时才会生效。这是初学者常遇到的"为什么 stretch 不生效"的原因。

align-items 交互演示（MDN）

设置 flex/grid 容器交叉轴方向项目的对齐方式。

html

<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>align-items 属性演示 - MDN 示例</title>
    <meta name="description" content="演示align（items 属性演示）效果。" />
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }
      body {
        font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
        min-height: 100vh;
        padding: 0;
      }
      .demo-layout {
        display: flex;
        height: 100vh;
        gap: 0;
      }
      .snippet-panel {
        width: 320px;
        flex-shrink: 0;
        padding: 16px;
        display: flex;
        flex-direction: column;
        gap: 8px;
        overflow-y: auto;
        border-right: 1px solid #ddd;
        background: #fafafa;
      }
      .snippet-btn {
        padding: 10px 14px;
        border: 1px solid #ccc;
        border-radius: 6px;
        background: #fff;
        font-family: "JetBrains Mono", "Fira Code", monospace;
        font-size: 13px;
        color: #333;
        cursor: pointer;
        text-align: left;
        transition: all 0.2s;
        line-height: 1.4;
      }
      .snippet-btn:hover {
        border-color: #8083ff;
        background: #f0f0ff;
      }
      .snippet-btn.active {
        border-color: #8083ff;
        background: #e8e8ff;
        color: #571bc1;
        font-weight: 600;
      }
      .preview-panel {
        flex: 1;
        padding: 16px;
        display: flex;
        align-items: center;
        justify-content: center;
        background: #fff;
        overflow: auto;
      }
      .preview-panel > section,
      .preview-panel > div:not(.snippet-panel):not(.demo-layout) {
        flex: 1;
        width: 100%;
        min-height: 0;
        height: 100%;
        display: flex;
        align-items: center;
        justify-content: center;
      }
      #example-element {
        border: 1px solid #c5c5c5;
        display: grid;
        width: 200px;
        grid-template-columns: 1fr 1fr;
        grid-auto-rows: 80px;
        grid-gap: 10px;
      }

      #example-element > div {
        background-color: rgba(0, 0, 255, 0.2);
        border: 3px solid blue;
      }
    </style>
  </head>
  <body>
    <div class="demo-layout">
      <div class="snippet-panel">
        <button class="snippet-btn active" data-index="0">align-items: stretch;</button>
        <button class="snippet-btn" data-index="1">align-items: center;</button>
        <button class="snippet-btn" data-index="2">align-items: start;</button>
        <button class="snippet-btn" data-index="3">align-items: end;</button>
      </div>
      <div class="preview-panel">
        <section class="default-example" id="default-example">
          <div class="example-container">
            <div class="transition-all" id="example-element">
              <div>One</div>
              <div>Two</div>
              <div>Three</div>
            </div>
          </div>
        </section>
      </div>
    </div>
    <script>
      const snippets = [
        `#example-element {
  align-items: stretch;
}`,
        `#example-element {
  align-items: center;
}`,
        `#example-element {
  align-items: start;
}`,
        `#example-element {
  align-items: end;
}`,
      ];

      let styleEl = document.createElement("style");
      document.head.appendChild(styleEl);

      function applySnippet(index) {
        styleEl.textContent = snippets[index];
        document.querySelectorAll(".snippet-btn").forEach((btn, i) => {
          btn.classList.toggle("active", i === index);
        });
      }

      document.querySelectorAll(".snippet-btn").forEach((btn) => {
        btn.addEventListener("click", () => applySnippet(parseInt(btn.dataset.index)));
      });

      applySnippet(0);
    </script>
  </body>
</html>

align-content

align-content 控制多行项目在交叉轴方向的整体对齐方式。仅在 flex-wrap: wrap 且有多行时生效，单行情况下无效。

值	行为
`stretch`	默认值，各行拉伸填满交叉轴剩余空间
`flex-start`	所有行紧贴交叉轴起点
`flex-end`	所有行紧贴交叉轴终点
`center`	所有行在交叉轴居中
`space-between`	行间间距相等，首尾行紧贴容器边缘
`space-around`	每行两侧间距相等

html

<div class="container">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
  <div class="item">4</div>
  <div class="item">5</div>
  <div class="item">6</div>
</div>

css

.container {
  display: flex;
  flex-wrap: wrap;           /* 必须允许换行 */
  align-content: space-between; /* 多行在交叉轴两端对齐 */
  height: 300px;             /* 容器必须有足够高度 */
  gap: 10px;
  border: 2px solid #1976d2;
}

.item {
  width: 120px;
  height: 60px;
  background: #e3f2fd;
  border: 1px solid #1976d2;
}

align-content 与 align-items 的区别：align-items 控制单行内项目在交叉轴上的对齐；align-content 控制多行作为整体在交叉轴上的分布。两者可以同时使用，align-content 先决定行的位置，align-items 再决定行内项目的对齐。

align-content 交互演示（MDN）

设置多行 flex/grid 项目在交叉轴上的分布方式。

html

<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>align-content 属性演示 - MDN 示例</title>
    <meta name="description" content="演示align（content 属性演示）效果。" />
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }
      body {
        font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
        min-height: 100vh;
        padding: 0;
      }
      .demo-layout {
        display: flex;
        height: 100vh;
        gap: 0;
      }
      .snippet-panel {
        width: 320px;
        flex-shrink: 0;
        padding: 16px;
        display: flex;
        flex-direction: column;
        gap: 8px;
        overflow-y: auto;
        border-right: 1px solid #ddd;
        background: #fafafa;
      }
      .snippet-btn {
        padding: 10px 14px;
        border: 1px solid #ccc;
        border-radius: 6px;
        background: #fff;
        font-family: "JetBrains Mono", "Fira Code", monospace;
        font-size: 13px;
        color: #333;
        cursor: pointer;
        text-align: left;
        transition: all 0.2s;
        line-height: 1.4;
      }
      .snippet-btn:hover {
        border-color: #8083ff;
        background: #f0f0ff;
      }
      .snippet-btn.active {
        border-color: #8083ff;
        background: #e8e8ff;
        color: #571bc1;
        font-weight: 600;
      }
      .preview-panel {
        flex: 1;
        padding: 16px;
        display: flex;
        align-items: center;
        justify-content: center;
        background: #fff;
        overflow: auto;
      }
      .preview-panel > section,
      .preview-panel > div:not(.snippet-panel):not(.demo-layout) {
        flex: 1;
        width: 100%;
        min-height: 0;
        height: 100%;
        display: flex;
        align-items: center;
        justify-content: center;
      }
      #example-element {
        border: 1px solid #c5c5c5;
        display: grid;
        grid-template-columns: 60px 60px;
        grid-auto-rows: 40px;
        column-gap: 10px;
        height: 180px;
      }

      #example-element > div {
        background-color: rgba(0, 0, 255, 0.2);
        border: 3px solid blue;
      }
    </style>
  </head>
  <body>
    <div class="demo-layout">
      <div class="snippet-panel">
        <button class="snippet-btn active" data-index="0">align-content: start;</button>
        <button class="snippet-btn" data-index="1">align-content: center;</button>
        <button class="snippet-btn" data-index="2">align-content: space-between;</button>
        <button class="snippet-btn" data-index="3">align-content: space-around;</button>
      </div>
      <div class="preview-panel">
        <section class="default-example" id="default-example">
          <div class="example-container">
            <div class="transition-all" id="example-element">
              <div>One</div>
              <div>Two</div>
              <div>Three</div>
            </div>
          </div>
        </section>
      </div>
    </div>
    <script>
      const snippets = [
        `#example-element {
  align-content: start;
}`,
        `#example-element {
  align-content: center;
}`,
        `#example-element {
  align-content: space-between;
}`,
        `#example-element {
  align-content: space-around;
}`,
      ];

      let styleEl = document.createElement("style");
      document.head.appendChild(styleEl);

      function applySnippet(index) {
        styleEl.textContent = snippets[index];
        document.querySelectorAll(".snippet-btn").forEach((btn, i) => {
          btn.classList.toggle("active", i === index);
        });
      }

      document.querySelectorAll(".snippet-btn").forEach((btn) => {
        btn.addEventListener("click", () => applySnippet(parseInt(btn.dataset.index)));
      });

      applySnippet(0);
    </script>
  </body>
</html>

gap

gap 属性控制 Flex 项目之间的间距，是替代传统 margin 方案的现代方式。它只作用于项目之间，不会在容器边缘产生间距。

属性	说明
`gap`	行间距和列间距的简写
`row-gap`	仅控制行间距（多行时生效）
`column-gap`	仅控制列间距

css

.container {
  display: flex;
  flex-wrap: wrap;

  /* 单值：行间距 = 列间距 */
  gap: 20px;

  /* 双值：行间距 列间距 */
  gap: 20px 30px;

  /* 也可以分别设置 */
  row-gap: 20px;
  column-gap: 30px;
}

html

<div class="container">
  <div class="item">A</div>
  <div class="item">B</div>
  <div class="item">C</div>
  <div class="item">D</div>
</div>

css

.container {
  display: flex;
  flex-wrap: wrap;
  gap: 16px; /* 项目间距 16px，不会在容器边缘产生间距 */
  padding: 16px;
  border: 2px solid #1976d2;
}

.item {
  flex: 1 1 150px;
  height: 80px;
  background: #e3f2fd;
  border: 1px solid #1976d2;
}

gap 与 margin 的核心区别：gap 只在项目之间生效，首尾项目不会产生多余间距；margin 需要手动处理首尾元素的间距问题（如 :last-child { margin-right: 0 }）。优先使用 gap。

gap 交互演示（MDN）

设置 flex/grid 容器中项目之间的间距，可分别指定行距与列距。

html

<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>gap 属性演示 - MDN 示例</title>
    <meta name="description" content="演示gap 属性演示效果。" />
    <style>
      * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
          }
          body {
            font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
            min-height: 100vh;
            padding: 0;
          }
          .demo-layout {
            display: flex;
            height: 100vh;
            gap: 0;
          }
          .snippet-panel {
            width: 320px;
            flex-shrink: 0;
            padding: 16px;
            display: flex;
            flex-direction: column;
            gap: 8px;
            overflow-y: auto;
            border-right: 1px solid #ddd;
            background: #fafafa;
          }
          .snippet-btn {
            padding: 10px 14px;
            border: 1px solid #ccc;
            border-radius: 6px;
            background: #fff;
            font-family: 'JetBrains Mono', 'Fira Code', monospace;
            font-size: 13px;
            color: #333;
            cursor: pointer;
            text-align: left;
            transition: all 0.2s;
            line-height: 1.4;
          }
          .snippet-btn:hover {
            border-color: #8083ff;
            background: #f0f0ff;
          }
          .snippet-btn.active {
            border-color: #8083ff;
            background: #e8e8ff;
            color: #571bc1;
            font-weight: 600;
          }
          .preview-panel {
            flex: 1;
            padding: 16px;
            display: flex;
            align-items: center;
            justify-content: center;
            background: #fff;
            overflow: auto;
          }
          .preview-panel > section,
          .preview-panel > div:not(.snippet-panel):not(.demo-layout) {
            flex: 1;
            width: 100%;
            min-height: 0;
            height: 100%;
            display: flex;
            align-items: center;
            justify-content: center;
          }
      :root {
        --link-color: 0;
        --secondary-button-fill-color: 0;
        --secondary-button-border-color: 0;
        --secondary-button-text-color: 0;
        --primary-button-text-color: 0;
        --primary-button-fill-color: 0;
        --primary-button-fill-color-active: 0;
        --secondary-button-hover-fill-color: 0;
        --secondary-button-hover-border-color: 0;
        --error-code-color: 0;
        --heading-color: 0;
        --google-red-10: 0;
        --small-link-color: 0;
        --background-color: 0;
        --google-blue-600: 0;
        --google-gray-300: 0;
        --google-gray-700: 0;
        --text-color: 0;
        --google-blue-300: 0;
        --google-gray-50: 0;
        --google-gray-800: 0;
        --google-gray-100: 0;
      }

      /* Copyright 2017 The Chromium Authors
       * Use of this source code is governed by a BSD-style license that can be
       * found in the LICENSE file. */
      a {
        color: var(--link-color);
        text-decoration: none
      }

      .nav-wrapper .secondary-button {
        background: var(--secondary-button-fill-color);
        border: 1px solid var(--secondary-button-border-color);
        color: var(--secondary-button-text-color);
        float: none;
        margin: 0;
        padding: 8px 16px
      }

      .hidden {
        display: none
      }

      .icon {
        background-repeat: no-repeat;
        background-size: 100%;
        height: 72px;
        margin: 0 0 40px;
        width: 72px;
        -webkit-user-select: none;
        display: inline-block
      }

      @media (prefers-color-scheme: dark) {

      }

      button {
        border: 0;
        border-radius: 20px;
        box-sizing: border-box;
        color: var(--primary-button-text-color);
        cursor: pointer;
        float: right;
        font-size: .875em;
        margin: 0;
        padding: 8px 16px;
        transition: box-shadow 150ms cubic-bezier(0.4, 0, 0.2, 1);
        user-select: none
      }

      [dir='rtl'] button {
        float: left
      }

      .bad-clock button, .captive-portal button, .https-only button, .insecure-form button, .lookalike-url button, .main-frame-blocked button, .neterror button, .pdf button, .ssl button, .enterprise-block button, .enterprise-warn button, .managed-profile-required button, .safe-browsing-billing button, .supervised-user-verify button, .supervised-user-verify-subframe button {
        background: var(--primary-button-fill-color)
      }

      button:active {
        background: var(--primary-button-fill-color-active);
        outline: 0
      }

      #debugging {
        display: inline;
        overflow: auto
      }

      .debugging-content {
        line-height: 1em;
        margin-bottom: 0;
        margin-top: 1em
      }

      .debugging-content-fixed-width {
        display: block;
        font-family: monospace;
        font-size: 1.2em;
        margin-top: 0.5em
      }

      .debugging-title {
        font-weight: bold
      }

      #details {
        margin: 0 0 50px
      }

      #details {
        p:not(:first-of-type) {
        margin-top: 20px
      }

      }

      .secondary-button:active {
        border-color: white;
        box-shadow: 0 1px 2px 0 rgba(60, 64, 67, .3), 0 2px 6px 2px rgba(60, 64, 67, .15)
      }

      .secondary-button:hover {
        background: var(--secondary-button-hover-fill-color);
        border-color: var(--secondary-button-hover-border-color);
        text-decoration: none
      }

      .error-code {
        color: var(--error-code-color);
        font-size: .8em;
        margin-top: 12px;
        text-transform: uppercase
      }

      #error-debugging-info {
        font-size: 0.8em
      }

      h1 {
        color: var(--heading-color);
        font-size: 1.6em;
        font-weight: normal;
        line-height: 1.25em;
        margin-bottom: 16px;
        margin-top: 0;
        word-wrap: break-word
      }

      h2 {
        font-size: 1.2em;
        font-weight: normal
      }

      input[type=checkbox] {
        opacity: 0
      }

      input[type=checkbox]:focus ~ .checkbox::after {
        outline: -webkit-focus-ring-color auto 5px
      }

      .interstitial-wrapper {
        box-sizing: border-box;
        font-size: 1em;
        line-height: 1.6em;
        margin: 14vh auto 0;
        max-width: 600px;
        width: 100%
      }

      #main-message > p {
        display: inline
      }

      #extended-reporting-opt-in {
        font-size: .875em;
        margin-top: 32px
      }

      #extended-reporting-opt-in label {
        display: grid;
        grid-template-columns: 1.8em 1fr;
        position: relative
      }

      #enhanced-protection-message {
        border-radius: 20px;
        font-size: 1em;
        margin-top: 32px;
        padding: 10px 5px
      }

      #enhanced-protection-message a {
        color: var(--google-red-10)
      }

      #enhanced-protection-message label {
        display: grid;
        grid-template-columns: 2.5em 1fr;
        position: relative
      }

      #enhanced-protection-message div {
        margin: 0.5em
      }

      #enhanced-protection-message .icon {
        height: 1.5em;
        vertical-align: middle;
        width: 1.5em
      }

      .nav-wrapper {
        margin-top: 51px
      }

      .nav-wrapper::after {
        clear: both;
        content: '';
        display: table;
        width: 100%
      }

      .small-link {
        color: var(--small-link-color);
        font-size: .875em
      }

      .checkboxes {
        flex: 0 0 24px
      }

      .checkbox {
        --padding: .9em;
        background: transparent;
        display: block;
        height: 1em;
        left: -1em;
        padding-inline-start: var(--padding);
        position: absolute;
        right: 0;
        top: -.5em;
        width: 1em
      }

      .checkbox::after {
        border: 1px solid white;
        border-radius: 2px;
        content: '';
        height: 1em;
        left: var(--padding);
        position: absolute;
        top: var(--padding);
        width: 1em
      }

      .checkbox::before {
        background: transparent;
        border: 2px solid white;
        border-inline-end-width: 0;
        border-top-width: 0;
        content: '';
        height: .2em;
        left: calc(.3em + var(--padding));
        opacity: 0;
        position: absolute;
        top: calc(.3em + var(--padding));
        transform: rotate(-45deg);
        width: .5em
      }

      input[type=checkbox]:checked ~ .checkbox::before {
        opacity: 1
      }

      @media (max-width: 700px) {
        .interstitial-wrapper {
        .interstitial-wrapper {
        padding: 0 10%
      }

      #error-debugging-info {
        overflow: auto
      }

      }

      @media (max-width: 420px) {
        button, [dir='rtl'] {
        button, [dir='rtl'] button, .small-link float: none;
        font-size: .825em;
        font-weight: 500;
        margin: 0;
        width: 100%;
      button {
        padding: 16px 24px
      }

      #details {
        margin: 20px 0 20px 0
      }

      #details {
        p: not(:first-of-type) margin-top: 10px
      }

      .secondary-button: not(.hidden) {
        display: block
      }

      margin-top: 20px;
        text-align: center;
        width: 100%;
        .interstitial-wrapper {
        padding: 0 5%
      }

      #extended-reporting-opt-in {
        margin-top: 24px
      }

      #enhanced-protection-message {
        margin-top: 24px
      }

      .nav-wrapper {
        margin-top: 30px
      }

      }

      /**
       * Mobile specific styling.
       * Navigation buttons are anchored to the bottom of the screen.
       * Details message replaces the top content in its own scrollable area.
       */
      @media (max-width: 420px) {
        .nav- {
        .nav-wrapper .secondary-button {
        border: 0
      }

      margin: 16px 0 0;
        margin-inline-end: 0;
        padding-bottom: 16px;
        padding-top: 16px
      }

      @media (min-width: 240px) and (max-width: 420px) and (min-height: 401px), (min-width: 421px) and (min-height: 240px) and (max-height: 560px) {
        body .nav-wra {
        body .nav-wrapper background: var(--background-color);
        bottom: 0;
        box-shadow: 0 -12px 24px var(--background-color);
        left: 0;
        margin: 0 auto;
        max-width: 736px;
        padding-inline-end: 24px;
        padding-inline-start: 24px;
        position: fixed;
        right: 0;
        width: 100%;
        z-index: 2;
        .interstitial-wrapper {
        max-width: 736px
      }

      #details, #main-content padding-bottom: 40px;
        #details {
        padding-top: 5.5vh
      }

      button.small-link color: var(--google-blue-600)
      }

      @media (max-width: 420px) and (orientation: portrait), (max-height: 560px) {
        button, [dir='rtl'] but {
        button, [dir='rtl'] button, button.small-link, .nav-wrapper .secondary-button font-family: Roboto-Regular,Helvetica;
        font-size: .933em;
        margin: 6px 0;
        transform: translatez(0);
        .nav-wrapper {
        box-sizing: border-box
      }

      padding-bottom: 8px;
        width: 100%;
        #details {
        box-sizing: border-box
      }

      height: auto;
        margin: 0;
        opacity: 1;
        transition: opacity 250ms cubic-bezier(0.4, 0, 0.2, 1);
        #details.hidden, #main-content.hidden height: 0;
        opacity: 0;
        overflow: hidden;
        padding-bottom: 0;
        transition: none;
      h1 {
        font-size: 1.5em
      }

      margin-bottom: 8px;
        .icon {
        margin-bottom: 5.69vh
      }

      .interstitial-wrapper {
        box-sizing: border-box
      }

      margin: 7vh auto 12px;
        padding: 0 24px;
        position: relative;
        .interstitial-wrapper p font-size: .95em;
        line-height: 1.61em;
        margin-top: 8px;
        #main-content {
        margin: 0
      }

      transition: opacity 100ms cubic-bezier(0.4, 0, 0.2, 1);
        .small-link {
        border: 0
      }

      .suggested-left > #control-buttons, .suggested-right > #control-buttons float: none;
        margin: 0
      }

      @media (min-width: 421px) and (min-height: 500px) and (max-height: 560px) {
        .interstitial-wrapper {
        .interstitial-wrapper {
        margin-top: 10vh
      }

      }

      @media (min-height: 400px) and (orientation: portrait) {
        .interstitial-wrapper {
        .interstitial-wrapper {
        margin-bottom: 145px
      }

      }

      @media (min-height: 299px) {
        .nav-wrapper {
        .nav-wrapper {
        padding-bottom: 16px
      }

      }

      @media (max-height: 560px) and (min-height: 240px) and (orientation: landscape) {
        .exte {
        .extended-reporting-has-checkbox #details padding-bottom: 80px
      }

      @media (min-height: 500px) and (max-height: 650px) and (max-width: 414px) and (orientation: portrait) {
        .interstitial-wrapper {
        .interstitial-wrapper {
        margin-top: 7vh
      }

      }

      @media (min-height: 650px) and (max-width: 414px) and (orientation: portrait) {
        .interstitial-wrapper {
        .interstitial-wrapper {
        margin-top: 10vh
      }

      }

      @media (max-height: 400px) and (orientation: portrait), (max-height: 239px) and (orientation: landscape), (max-width: 419px) and (max-height: 399px) {
        .interstitial-wrapper {
        .interstitial-wrapper {
        display: flex
      }

      flex-direction: column;
        margin-bottom: 0;
        #details {
        flex: 1 1 auto
      }

      order: 0;
        #main-content {
        flex: 1 1 auto
      }

      order: 0;
        .nav-wrapper {
        flex: 0 1 auto
      }

      margin-top: 8px;
        order: 1;
        padding-inline-end: 0;
        padding-inline-start: 0;
        position: relative;
        width: 100%;
        button, .nav-wrapper .secondary-button padding: 16px 24px;
        button.small-link color: var(--google-blue-600)
      }

      @media (max-width: 239px) and (orientation: portrait) {
        .nav-wrapper {
        .nav-wrapper {
        padding-inline-end: 0
      }

      padding-inline-start: 0
      }

      /* Copyright 2013 The Chromium Authors
       * Use of this source code is governed by a BSD-style license that can be
       * found in the LICENSE file. */
      html[subframe] #main-frame-error {
        display: none
      }

      html:not([subframe]) #sub-frame-error {
        display: none
      }

      h1 span {
        font-weight: 500
      }

      .icon-generic {
        content: image-set( url(data:image/png;
        base64,iVBORw0KGgoAAAANSUhEUgAAAEgAAABIAQMAAABvIyEEAAAABlBMVEUAAABTU1OoaSf/AAAAAXRSTlMAQObYZgAAAENJREFUeF7tzbEJACEQRNGBLeAasBCza2lLEGx0CxFGG9hBMDDxRy/72O9FMnIFapGylsu1fgoBdkXfUHLrQgdfrlJN1BdYBjQQm3UAAAAASUVORK5CYII=) 1x, url(data: image/png;
        base64,iVBORw0KGgoAAAANSUhEUgAAAJAAAACQAQMAAADdiHD7AAAABlBMVEUAAABTU1OoaSf/AAAAAXRSTlMAQObYZgAAAFJJREFUeF7t0cENgDAMQ9FwYgxG6WjpaIzCCAxQxVggFuDiCvlLOeRdHR9yzjncHVoq3npu+wQUrUuJHylSTmBaespJyJQoObUeyxDQb3bEm5Au81c0pSCD8HYAAAAASUVORK5CYII=) 2x)
      }

      .icon-info {
        content: image-set( url(data:image/png;
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      #suggestions-list p {
        margin-block-end: 0
      }

      #suggestions-list ul {
        margin-top: 0
      }

      .single-suggestion {
        list-style-type: none;
        padding-inline-start: 0
      }

      .link-button {
        color: rgb(66, 133, 244);
        display: inline-block;
        font-weight: bold;
        text-transform: uppercase
      }

      #sub-frame-error-details {
        c {
        color: #8F8F8F;
        text-shadow: 0 1px 0 rgba(255,255,255,0.3)
      }

      .secondary-button {
        background: #d9d9d9;
        color: #696969;
        margin-inline-end: 16px
      }

      .snackbar {
        background: #323232;
        border-radius: 2px;
        bottom: 24px;
        box-sizing: border-box;
        color: #fff;
        font-size: .87em;
        left: 24px;
        max-width: 568px;
        min-width: 288px;
        opacity: 0;
        padding: 16px 24px 12px;
        position: fixed;
        transform: translateY(90px);
        will-change: opacity, transform;
        z-index: 999
      }

      .snackbar-show {
        -webkit-animation: show-snackbar 250ms cubic-bezier(0, 0, 0.2, 1) forwards, hide-snackbar 250ms cubic-bezier(0.4, 0, 1, 1) forwards 5s
      }

      @-webkit-keyframes show-snackbar {
        100% {
        100% {
        opacity: 1
      }

      transform: translateY(0)
      }

      @-webkit-keyframes hide-snackbar {
        0% {
        0% {
        opacity: 1
      }

      transform: translateY(0);
        100% {
        opacity: 0
      }

      transform: translateY(90px)
      }

      .suggestions {
        margin-top: 18px
      }

      .suggestion-header {
        font-weight: bold;
        margin-bottom: 4px
      }

      .suggestion- @media (max-width: 640px), (max-height: 640px) {
        h1 {
        h1 {
        margin: 0 0 15px
      }

      .suggestions {
        margin-top: 10px
      }

      .suggestion-header {
        margin-bottom: 0
      }

      }

      #cancel-save-page-button {
        background-image: url(data:image/svg+xml;
        base64,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);
        background-position: right 27px center;
        background-repeat: no-repeat;
        border: 1px solid var(--google-gray-300);
        border-radius: 5px;
        color: var(--google-gray-700);
        margin-bottom: 26px;
        padding-bottom: 16px;
        padding-inline-end: 88px;
        padding-inline-start: 16px;
        padding-top: 16px;
        text-align: start
      }

      html[dir='rtl'] #cancel-save-page-button {
        background-position: left 27px center
      }

      #save-page-for-later-button {
        display: flex;
        justify-content: start
      }

      #save-page-for-later-button {
        a::before {
        content: url(data:image/svg+xml
      }

      base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIxLjJlbSIgaGVpZ2h0PSIxLjJlbSIgdmlld0JveD0iMCAwIDI0IDI0Ij48cGF0aCBkPSJNNSAyMGgxNHYtMkg1bTE0LTloLTRWM0g5djZINWw3IDcgNy03eiIgZmlsbD0iIzQyODVGNCIvPjwvc3ZnPg==);
        display: inline-block;
        margin-inline-end: 4px;
        vertical-align: -webkit-baseline-middle
      }

      .hidden#save-page-for-later-button {
        display: none
      }

      html[subframe] #sub-frame-error {
        -webkit-align-items: center;
        -webkit-flex-flow: column;
        -webkit-justify-content: center;
        background-color: #DDD;

        height: 100%;
        left: 0;
        position: absolute;
        text-align: center;
        top: 0;
        transition: background-color 200ms ease-in-out;
        width: 100%
      }

      #sub-frame-error:hover {
        background-color: #EEE
      }

      #sub-frame-error .icon-generic {
        margin: 0 0 16px
      }

      #sub-frame-error-details {
        margin: 0 10px;
        text-align: center;
        opacity: 0
      }

      #sub-frame-error:hover #sub-frame-error-details {
        opacity: 1
      }

      @media (max-width: 200px), (max-height: 95px) {
        #sub- {
        #sub-frame-error-details {
        display: none
      }

      }

      @media (max-height: 100px) {
        #sub- {
        #sub-frame-error .icon-generic height: auto;
        margin: 0;
        padding-top: 0;
        width: 25px
      }

      #details-button {
        box-shadow: none;
        min-width: 0
      }

      .suggested-left > #control-buttons, .suggested-right > #details-button {
        float: left
      }

      .suggested-right > #control-buttons, .suggested-left > #details-button {
        float: right
      }

      .suggested-left .secondary-button {
        margin-inline-end: 0;
        margin-inline-start: 16px
      }

      #details-button.singular {
        float: none
      }

      #download-button {
        padding-bottom: 4px;
        padding-top: 4px;
        position: relative
      }

      #download-button::before {
        background: image-set( url(data:image/png;
        base64,iVBORw0KGgoAAAANSUhEUgAAABgAAAAYCAQAAABKfvVzAAAAO0lEQVQ4y2NgGArgPxIY1YChsOE/LtBAmpYG0mxpIOSDBpKUo2lpIDZxNJCkHKqlYZAla3RAHQ1DFgAARRroHyLNTwwAAAAASUVORK5CYII=) 1x, url(data: image/png;
        base64,iVBORw0KGgoAAAANSUhEUgAAADAAAAAwCAQAAAD9CzEMAAAAZElEQVRYw+3Ruw3AMAwDUY3OzZUmRRD4E9iim9wNwAdbEURHyk4AAAAATiCVK8lLyPsKeT9K3lsownnunfkPxO78hKiYHxBV8x2icr5BVM+/CMf8g3DN34Rzns6ViwHUAUQ/6wIAd5Km7l6c8AAAAABJRU5ErkJggg==) 2x) no-repeat;
        content: '';
        display: inline-block;
        height: 24px;
        margin-inline-end: 4px;
        margin-inline-start: -4px;
        vertical-align: middle;
        width: 24px
      }

      #download-button:disabled {
        background: rgb(180, 206, 249);
        color: rgb(255, 255, 255)
      }

      #buttons::after {
        clear: both;
        content: '';
        display: block;
        width: 100%
      }

      html[dir='rtl'] .runner-container, html[dir='rtl'].offline .icon-offline {
        transform: scaleX(-1)
      }

      .offline {
        t {
        transition: filter 1.5s cubic-bezier(0.65, 0.05, 0.36, 1), background-color 1.5s cubic-bezier(0.65, 0.05, 0.36, 1);
        will-change: filter, background-color
      }

      .offline .offline #main-message > p {
        display: none
      }

      .offline.inverted {
        background-color: #fff;
        filter: invert(1)
      }

      .offline.inverted .offline .interstitial-wrapper {
        color: var(--text-color);
        font-size: 1em;
        line-height: 1.55;
        margin: 0 auto;
        max-width: 600px;
        padding-top: 100px;
        position: relative;
        width: 100%
      }

      .offline .runner-container {
        direction: ltr;
        height: 150px;
        max-width: 600px;
        overflow: hidden;
        position: absolute;
        top: 35px;
        width: 44px
      }

      .offline .runner-container:focus {
        outline: none
      }

      .offline .runner-container:focus-visible {
        outline: 3px solid var(--google-blue-300)
      }

      .offline .runner-canvas {
        height: 150px;
        max-width: 600px;
        opacity: 1;
        overflow: hidden;
        position: absolute;
        top: 0;
        z-index: 10
      }

      .offline .controller {
        height: 100vh;
        left: 0;
        position: absolute;
        top: 0;
        width: 100vw;
        z-index: 9
      }

      #offline-resources {
        display: none
      }

      #offline-instruction {
        image-rendering: pixelated;
        left: 0;
        margin: auto;
        position: absolute;
        right: 0;
        top: 60px;
        width: fit-content
      }

      .offline-runner-live-region {
        bottom: 0;
        clip-path: polygon(0 0, 0 0, 0 0);
        color: var(--background-color);
        display: block;
        font-size: xx-small;
        overflow: hidden;
        position: absolute;
        text-align: center;
        transition: color 1.5s cubic-bezier(0.65, 0.05, 0.36, 1);
        user-select: none
      }

      .slow-speed-option {
        align-items: center;
        background: var(--google-gray-50);
        border-radius: 24px/50%;
        bottom: 0;
        color: var(--error-code-color);
        display: inline-flex;
        font-size: 1em;
        left: 0;
        line-height: 1.1em;
        margin: 5px auto;
        padding: 2px 12px 3px 20px;
        position: absolute;
        right: 0;
        width: max-content;
        z-index: 999
      }

      .slow-speed-option.hidden {
        display: none
      }

      .slow-speed-option [type=checkbox] {
        opacity: 0;
        pointer-events: none;
        position: absolute
      }

      .slow-speed-option .slow-speed-toggle {
        cursor: pointer;
        margin-inline-start: 8px;
        padding: 8px 4px;
        position: relative
      }

      .slow-speed-option [type=checkbox]:disabled ~ .slow-speed-toggle {
        cursor: default
      }

      .slow-speed-option-label [type=checkbox] {
        opacity: 0;
        pointer-events: none;
        position: absolute
      }

      .slow-speed-option .slow-speed-toggle::before, .slow-speed-option .slow-speed-toggle::after {
        content: '';
        display: block;
        margin: 0 3px;
        transition: all 100ms cubic-bezier(0.4, 0, 1, 1)
      }

      .slow-speed-option .slow-speed-toggle::before {
        background: rgb(189,193,198);
        border-radius: 0.65em;
        height: 0.9em;
        width: 2em
      }

      .slow-speed-option .slow-speed-toggle::after {
        background: #fff;
        border-radius: 50%;
        box-shadow: 0 1px 3px 0 rgb(0 0 0 / 40%);
        height: 1.2em;
        position: absolute;
        top: 51%;
        transform: translate(-20%, -50%);
        width: 1.1em
      }

      .slow-speed-option [type=checkbox]:focus + .slow-speed-toggle {
        box-shadow: 0 0 8px rgb(94, 158, 214);
        outline: 1px solid rgb(93, 157, 213)
      }

      .slow-speed-option [type=checkbox]:checked + .slow-speed-toggle::before {
        background: var(--google-blue-600);
        opacity: 0.5
      }

      .slow-speed-option [type=checkbox]:checked + .slow-speed-toggle::after {
        background: var(--google-blue-600);
        transform: translate(calc(2em - 90%), -50%)
      }

      .slow-speed-option [type=checkbox]:checked:disabled + .slow-speed-toggle::before {
        background: rgb(189,193,198)
      }

      .slow-speed-option [type=checkbox]:checked:disabled + .slow-speed-toggle::after {
        background: var(--google-gray-50)
      }

      @media (max-width: 420px) {
        #download {
        #download-button {
        padding-bottom: 12px
      }

      padding-top: 12px;
        .suggested-left > #control-buttons, .suggested-right > #control-buttons float: none;
        .snackbar {
        border-radius: 0
      }

      bottom: 0;
        left: 0;
        width: 100%
      }

      @media (max-height: 350px) {
        h1 {
        ma {
        h1 {
        margin: 0 0 15px
      }

      .icon-offline {
        margin: 0 0 10px
      }

      .interstitial-wrapper {
        margin-top: 5%
      }

      .nav-wrapper {
        margin-top: 30px
      }

      }

      @media (min-width: 420px) and (max-width: 736px) and (min-height: 240px) and (max-height: 420px) and (orientation:landscape) {
        .interstitial-wrapper {
        .interstitial-wrapper {
        margin-bottom: 100px
      }

      }

      @media (max-width: 360px) and (max-height: 480px) {
        .offlin {
        .offline .interstitial-wrapper {
        padding-top: 60px
      }

      .offline .runner-container {
        top: 8px
      }

      }

      @media (min-height: 240px) and (orientation: landscape) {
        .offlin {
        .offline .interstitial-wrapper {
        margin-bottom: 90px
      }

      .icon-offline {
        margin-bottom: 20px
      }

      }

      @media (max-height: 320px) and (orientation: landscape) {
        .icon-offline {
        .icon-offline {
        margin-bottom: 0
      }

      .offline .runner-container {
        top: 10px
      }

      }

      @media (max-width: 240px) {
        button {
        button {
        padding-inline-end: 12px
      }

      padding-inline-start: 12px;
        .interstitial-wrapper {
        overflow: inherit
      }

      padding: 0 8px
      }

      @media (max-width: 120px) {
        butto {
        button {
        width: auto
      }

      }

      .arcade-mode, .arcade-mode .runner-container, .arcade-mode .runner-canvas {
        image-rendering: pixelated;
        max-width: 100%;
        overflow: hidden
      }

      .arcade-mode #buttons, .arcade-mode #main-content {
        opacity: 0;
        overflow: hidden
      }

      .arcade-mode .interstitial-wrapper {
        height: 100vh;
        max-width: 100%;
        overflow: hidden
      }

      .arcade-mode .runner-container {
        left: 0;
        margin: auto;
        right: 0;
        transform-origin: top center;
        transition: transform 250ms cubic-bezier(0.4, 0, 1, 1) 400ms;
        z-index: 2
      }

      @media (prefers-color-scheme: dark) {
        .icon {
        filter: {
        .icon {
        filter: invert(1)
      }

      .offline .runner-canvas {
        filter: invert(1)
      }

      .offline.inverted {
        background-color: var(--background-color)
      }

      filter: invert(0);
        .offline.inverted .offline.inverted .offline-runner-live-region color: #fff;
        #suggestions-list a color: var(--link-color);
        .slow-speed-option {
        background: var(--google-gray-800)
      }

      color: var(--google-gray-100);
        .slow-speed-option .slow-speed-toggle::before, .slow-speed-option [type=checkbox]:checked:disabled + .slow-speed-toggle::before background: rgb(189,193,198);
        .slow-speed-option [type=checkbox]: checked + .slow-speed-toggle::after, .slow-speed-option [type=checkbox]:checked + .slow-speed-toggle::before {
        background: var(--google-blue-300)
      }

      }

      #main-frame-error:not(.showing-details) #details {
        display: none
      }

      @media (min-width: 240px) and (max-width: 420px) and (min-height: 401px), (min-height: 240px) and (max-height: 560px) and (min-width: 421px) {
        #main {
        #main-frame-error.showing-details #main-content, #main-frame-error.showing-details .runner-container display: none
      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }

      }
    </style>
  </head>
  <body>
    <div class="demo-layout">
      <div class="snippet-panel">
        <button class="snippet-btn active" data-index="0">gap: 0;</button>
        <button class="snippet-btn" data-index="1">gap: 10%;</button>
        <button class="snippet-btn" data-index="2">gap: 1em;</button>
        <button class="snippet-btn" data-index="3">gap: 10px 20px;</button>
        <button class="snippet-btn" data-index="4">gap: calc(20px + 10%);</button>
      </div>
      <div class="preview-panel">
        <section class="default-example" id="default-example">
          <div class="example-container">
            <div class="transition-all" id="example-element">
              <div>One</div>
              <div>Two</div>
              <div>Three</div>
              <div>Four</div>
              <div>Five</div>
            </div>
          </div>
        </section>
      </div>
    </div>
    <script>
      const snippets = [
        `#example-element {
  gap: 0;
}`,
        `#example-element {
  gap: 10%;
}`,
        `#example-element {
  gap: 1em;
}`,
        `#example-element {
  gap: 10px 20px;
}`,
        `#example-element {
  gap: calc(20px + 10%);
}`,
      ];

      let styleEl = document.createElement("style");
      document.head.appendChild(styleEl);

      function applySnippet(index) {
        styleEl.textContent = snippets[index];
        document.querySelectorAll(".snippet-btn").forEach((btn, i) => {
          btn.classList.toggle("active", i === index);
        });
      }

      document.querySelectorAll(".snippet-btn").forEach((btn) => {
        btn.addEventListener("click", () => applySnippet(parseInt(btn.dataset.index)));
      });

      applySnippet(0);
    </script>
  </body>
</html>

row-gap 交互演示（MDN）

设置多行布局中行与行之间的间距。

html

<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>row-gap 属性演示 - MDN 示例</title>
    <meta name="description" content="演示row（gap 属性演示）效果。" />
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }
      body {
        font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
        min-height: 100vh;
        padding: 0;
      }
      .demo-layout {
        display: flex;
        height: 100vh;
        gap: 0;
      }
      .snippet-panel {
        width: 320px;
        flex-shrink: 0;
        padding: 16px;
        display: flex;
        flex-direction: column;
        gap: 8px;
        overflow-y: auto;
        border-right: 1px solid #ddd;
        background: #fafafa;
      }
      .snippet-btn {
        padding: 10px 14px;
        border: 1px solid #ccc;
        border-radius: 6px;
        background: #fff;
        font-family: "JetBrains Mono", "Fira Code", monospace;
        font-size: 13px;
        color: #333;
        cursor: pointer;
        text-align: left;
        transition: all 0.2s;
        line-height: 1.4;
      }
      .snippet-btn:hover {
        border-color: #8083ff;
        background: #f0f0ff;
      }
      .snippet-btn.active {
        border-color: #8083ff;
        background: #e8e8ff;
        color: #571bc1;
        font-weight: 600;
      }
      .preview-panel {
        flex: 1;
        padding: 16px;
        display: flex;
        align-items: center;
        justify-content: center;
        background: #fff;
        overflow: auto;
      }
      .preview-panel > section,
      .preview-panel > div:not(.snippet-panel):not(.demo-layout) {
        flex: 1;
        width: 100%;
        min-height: 0;
        height: 100%;
        display: flex;
        align-items: center;
        justify-content: center;
      }
      #example-element {
        border: 1px solid #c5c5c5;
        display: grid;
        grid-template-columns: 1fr 1fr;
        width: 200px;
      }

      #example-element > div {
        background-color: rgba(0, 0, 255, 0.2);
        border: 3px solid blue;
      }
    </style>
  </head>
  <body>
    <div class="demo-layout">
      <div class="snippet-panel">
        <button class="snippet-btn active" data-index="0">row-gap: 0;</button>
        <button class="snippet-btn" data-index="1">row-gap: 1ch;</button>
        <button class="snippet-btn" data-index="2">row-gap: 1em;</button>
        <button class="snippet-btn" data-index="3">row-gap: 20px;</button>
      </div>
      <div class="preview-panel">
        <section class="default-example" id="default-example">
          <div class="example-container">
            <div class="transition-all" id="example-element">
              <div>One</div>
              <div>Two</div>
              <div>Three</div>
              <div>Four</div>
              <div>Five</div>
            </div>
          </div>
        </section>
      </div>
    </div>
    <script>
      const snippets = [
        `#example-element {
  row-gap: 0;
}`,
        `#example-element {
  row-gap: 1ch;
}`,
        `#example-element {
  row-gap: 1em;
}`,
        `#example-element {
  row-gap: 20px;
}`,
      ];

      let styleEl = document.createElement("style");
      document.head.appendChild(styleEl);

      function applySnippet(index) {
        styleEl.textContent = snippets[index];
        document.querySelectorAll(".snippet-btn").forEach((btn, i) => {
          btn.classList.toggle("active", i === index);
        });
      }

      document.querySelectorAll(".snippet-btn").forEach((btn) => {
        btn.addEventListener("click", () => applySnippet(parseInt(btn.dataset.index)));
      });

      applySnippet(0);
    </script>
  </body>
</html>

项目属性详解

Flex 项目属性控制单个项目的伸缩行为、排列顺序和对齐覆盖。以下属性均设置在 Flex 项目上。

flex-grow

flex-grow 定义项目的扩展比例，决定当容器有剩余空间时，项目如何瓜分多余的空间。默认值为 0，即不扩展。

值为 0：不扩展，保持原始尺寸
值为正整数：按比例瓜分剩余空间
值不支持负数

html

<div class="container">
  <div class="item grow-1">A</div>
  <div class="item grow-2">B</div>
  <div class="item">C</div>
</div>

css

.container {
  display: flex;
  border: 2px solid #1976d2;
}

.item {
  width: 80px;
  height: 40px;
  background: #e3f2fd;
  border: 1px solid #1976d2;
}

/* A 扩展比例 1，B 扩展比例 2，C 不扩展 */
/* 剩余空间按 1:2 分配给 A 和 B */
.grow-1 { flex-grow: 1; }
.grow-2 { flex-grow: 2; }

计算方式：假设容器剩余空间为 S，三个项目的 flex-grow 分别为 1、2、0，则 A 获得 S × 1/3 的额外空间，B 获得 S × 2/3 的额外空间，C 不获得。

注意：flex-grow 分配的是剩余空间，不是容器的全部空间。剩余空间 = 容器主轴尺寸 - 所有项目的 flex-basis 之和。如果项目已经占满容器，flex-grow 不会生效。

flex-grow 交互演示（MDN）

设置 flex 项目在有剩余空间时的放大比例。

html

<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>flex-grow 属性演示 - MDN 示例</title>
    <meta name="description" content="演示flex（grow 属性演示）效果。" />
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }
      body {
        font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
        min-height: 100vh;
        padding: 0;
      }
      .demo-layout {
        display: flex;
        height: 100vh;
        gap: 0;
      }
      .snippet-panel {
        width: 320px;
        flex-shrink: 0;
        padding: 16px;
        display: flex;
        flex-direction: column;
        gap: 8px;
        overflow-y: auto;
        border-right: 1px solid #ddd;
        background: #fafafa;
      }
      .snippet-btn {
        padding: 10px 14px;
        border: 1px solid #ccc;
        border-radius: 6px;
        background: #fff;
        font-family: "JetBrains Mono", "Fira Code", monospace;
        font-size: 13px;
        color: #333;
        cursor: pointer;
        text-align: left;
        transition: all 0.2s;
        line-height: 1.4;
      }
      .snippet-btn:hover {
        border-color: #8083ff;
        background: #f0f0ff;
      }
      .snippet-btn.active {
        border-color: #8083ff;
        background: #e8e8ff;
        color: #571bc1;
        font-weight: 600;
      }
      .preview-panel {
        flex: 1;
        padding: 16px;
        display: flex;
        align-items: center;
        justify-content: center;
        background: #fff;
        overflow: auto;
      }
      .preview-panel > section,
      .preview-panel > div:not(.snippet-panel):not(.demo-layout) {
        flex: 1;
        width: 100%;
        min-height: 0;
        height: 100%;
        display: flex;
        align-items: center;
        justify-content: center;
      }
      .default-example {
        border: 1px solid #c5c5c5;
        width: auto;
        max-height: 300px;
        display: flex;
      }

      .default-example > div {
        background-color: rgba(0, 0, 255, 0.2);
        border: 3px solid blue;
        margin: 10px;
        flex-grow: 1;
        flex-shrink: 1;
        flex-basis: 0;
      }
    </style>
  </head>
  <body>
    <div class="demo-layout">
      <div class="snippet-panel">
        <button class="snippet-btn active" data-index="0">flex-g行: 1;</button>
        <button class="snippet-btn" data-index="1">flex-g行: 2;</button>
        <button class="snippet-btn" data-index="2">flex-g行: 3;</button>
      </div>
      <div class="preview-panel">
        <section class="default-example" id="default-example">
          <div class="transition-all" id="example-element">I grow</div>
          <div>Item Two</div>
          <div>Item Three</div>
        </section>
      </div>
    </div>
    <script>
      const snippets = [
        `#example-element {
  flex-grow: 1;
}`,
        `#example-element {
  flex-grow: 2;
}`,
        `#example-element {
  flex-grow: 3;
}`,
      ];

      let styleEl = document.createElement("style");
      document.head.appendChild(styleEl);

      function applySnippet(index) {
        styleEl.textContent = snippets[index];
        document.querySelectorAll(".snippet-btn").forEach((btn, i) => {
          btn.classList.toggle("active", i === index);
        });
      }

      document.querySelectorAll(".snippet-btn").forEach((btn) => {
        btn.addEventListener("click", () => applySnippet(parseInt(btn.dataset.index)));
      });

      applySnippet(0);
    </script>
  </body>
</html>

flex-shrink

flex-shrink 定义项目的收缩比例，决定当项目总宽度超出容器时，各项目如何缩减尺寸。默认值为 1，即等比收缩。

值为 0：不收缩，保持原始尺寸
值为正整数：按比例收缩
值不支持负数

html

<div class="container">
  <div class="item shrink-0">A</div>
  <div class="item">B</div>
  <div class="item">C</div>
</div>

css

.container {
  display: flex;
  width: 300px; /* 容器宽度有限 */
  border: 2px solid #1976d2;
}

.item {
  width: 150px; /* 三个项目总宽 450px，超出容器 */
  height: 40px;
  background: #e3f2fd;
  border: 1px solid #1976d2;
}

/* A 不收缩，B 和 C 等比收缩 */
.shrink-0 { flex-shrink: 0; }

计算方式：收缩量的分配不仅看 flex-shrink 值，还要考虑项目的 flex-basis。具体公式为：项目收缩量 = 溢出空间 × (该项目的 shrink × basis / 所有项目的 shrink × basis 之和)。这意味着尺寸较大的项目会收缩更多。

常见问题：当项目内容（如长文本）撑开宽度导致溢出时，设置 flex-shrink: 0 可以阻止项目被压缩，但需要配合 overflow: hidden 或 min-width: 0 来处理溢出内容。

flex-basis

flex-basis 定义项目在主轴上的初始尺寸，即浏览器在分配剩余空间之前所认定的项目大小。默认值为 auto。

值	行为
`auto`	使用项目的 `width`（或 `height`），若也未设置则由内容撑开
`0`	初始尺寸为 0，所有空间都作为剩余空间按 `flex-grow` 分配
具体长度	如 `200px`、`30%`，使用指定值

html

<div class="container">
  <div class="item basis-200">A</div>
  <div class="item basis-auto">B</div>
  <div class="item basis-0">C</div>
</div>

css

.container {
  display: flex;
  border: 2px solid #1976d2;
}

.item {
  height: 40px;
  flex-grow: 1; /* 都参与扩展 */
  background: #e3f2fd;
  border: 1px solid #1976d2;
}

/* 初始尺寸 200px，再参与扩展 */
.basis-200 { flex-basis: 200px; }

/* 初始尺寸由内容决定 */
.basis-auto { flex-basis: auto; }

/* 初始尺寸为 0，完全按 flex-grow 比例分配空间 */
.basis-0 { flex-basis: 0; }

flex-basis 与 width 的优先级：在 Flex 布局中，flex-basis 的优先级高于 width（主轴为水平时）。当 flex-basis 为 auto 时，才会回退使用 width。但 min-width 和 max-width 始终有效，会约束 flex-basis 的计算结果。

flex-basis 交互演示（MDN）

设置 flex 项目在主轴上的初始尺寸。

html

<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>flex-basis 属性演示 - MDN 示例</title>
    <meta name="description" content="演示flex（basis 属性演示）效果。" />
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }
      body {
        font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
        min-height: 100vh;
        padding: 0;
      }
      .demo-layout {
        display: flex;
        height: 100vh;
        gap: 0;
      }
      .snippet-panel {
        width: 320px;
        flex-shrink: 0;
        padding: 16px;
        display: flex;
        flex-direction: column;
        gap: 8px;
        overflow-y: auto;
        border-right: 1px solid #ddd;
        background: #fafafa;
      }
      .snippet-btn {
        padding: 10px 14px;
        border: 1px solid #ccc;
        border-radius: 6px;
        background: #fff;
        font-family: "JetBrains Mono", "Fira Code", monospace;
        font-size: 13px;
        color: #333;
        cursor: pointer;
        text-align: left;
        transition: all 0.2s;
        line-height: 1.4;
      }
      .snippet-btn:hover {
        border-color: #8083ff;
        background: #f0f0ff;
      }
      .snippet-btn.active {
        border-color: #8083ff;
        background: #e8e8ff;
        color: #571bc1;
        font-weight: 600;
      }
      .preview-panel {
        flex: 1;
        padding: 16px;
        display: flex;
        align-items: center;
        justify-content: center;
        background: #fff;
        overflow: auto;
      }
      .preview-panel > section,
      .preview-panel > div:not(.snippet-panel):not(.demo-layout) {
        flex: 1;
        width: 100%;
        min-height: 0;
        height: 100%;
        display: flex;
        align-items: center;
        justify-content: center;
      }
      .default-example {
        border: 1px solid #c5c5c5;
        width: auto;
        max-height: 300px;
        display: flex;
      }

      .default-example > div {
        background-color: rgba(0, 0, 255, 0.2);
        border: 3px solid blue;
        margin: 10px;
        flex-grow: 1;
        flex-shrink: 1;
        flex-basis: auto;
      }
    </style>
  </head>
  <body>
    <div class="demo-layout">
      <div class="snippet-panel">
        <button class="snippet-btn active" data-index="0">flex-basis: auto;</button>
        <button class="snippet-btn" data-index="1">flex-basis: 0;</button>
        <button class="snippet-btn" data-index="2">flex-basis: 200px;</button>
      </div>
      <div class="preview-panel">
        <section class="default-example" id="default-example">
          <div class="transition-all" id="example-element">Item One</div>
          <div>Item Two</div>
          <div>Item Three</div>
        </section>
      </div>
    </div>
    <script>
      const snippets = [
        `#example-element {
  flex-basis: auto;
}`,
        `#example-element {
  flex-basis: 0;
}`,
        `#example-element {
  flex-basis: 200px;
}`,
      ];

      let styleEl = document.createElement("style");
      document.head.appendChild(styleEl);

      function applySnippet(index) {
        styleEl.textContent = snippets[index];
        document.querySelectorAll(".snippet-btn").forEach((btn, i) => {
          btn.classList.toggle("active", i === index);
        });
      }

      document.querySelectorAll(".snippet-btn").forEach((btn) => {
        btn.addEventListener("click", () => applySnippet(parseInt(btn.dataset.index)));
      });

      applySnippet(0);
    </script>
  </body>
</html>

flex 简写

flex 是 flex-grow、flex-shrink 和 flex-basis 的简写属性，推荐优先使用简写而非单独设置三个属性。默认值为 0 1 auto。

css

.item {
  /* 完整语法 */
  flex: <flex-grow> <flex-shrink>? <flex-basis>?;

  /* 常见值 */
  flex: 0 1 auto;   /* 默认值：不扩展，可收缩，尺寸由内容决定 */
  flex: 1;          /* 等同于 flex: 1 1 0% —— 等分空间 */
  flex: auto;       /* 等同于 flex: 1 1 auto —— 弹性填充 */
  flex: none;       /* 等同于 flex: 0 0 auto —— 固定大小，不伸缩 */
}

常见值对照表：

值	等同于	效果	典型用途
`flex: initial`	`0 1 auto`	不扩展，可收缩	默认行为，内容自适应
`flex: auto`	`1 1 auto`	可扩展可收缩	弹性填充，保留内容最小宽度
`flex: none`	`0 0 auto`	固定大小，不伸缩	固定宽度侧边栏、图标
`flex: 1`	`1 1 0%`	等分空间	等宽列布局
`flex: 2`	`2 1 0%`	占 2 份空间	按比例分配
`flex: 1 0 200px`	—	最小 200px，可扩展	弹性最小宽度

html

<div class="container">
  <div class="sidebar">侧边栏</div>
  <div class="main">主内容</div>
</div>

css

.container {
  display: flex;
  height: 100vh;
}

/* 侧边栏固定 250px，不伸缩 */
.sidebar {
  flex: none;
  width: 250px;
  background: #e3f2fd;
}

/* 主内容区占据剩余空间 */
.main {
  flex: 1; /* 等同于 flex: 1 1 0% */
  background: #fff3e0;
}

flex: 1 与 flex: auto 的关键区别：flex: 1 的 flex-basis 为 0%，所有空间都按 flex-grow 比例分配，实现严格等分；flex: auto 的 flex-basis 为 auto，先保留内容固有尺寸，再分配剩余空间，适合内容宽度差异较大的场景。

flex 交互演示（MDN）

flex-grow/flex-shrink/flex-basis 的简写属性。

html

<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>flex 属性演示 - MDN 示例</title>
    <meta name="description" content="演示flex 属性演示效果。" />
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }
      body {
        font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
        min-height: 100vh;
        padding: 0;
      }
      .demo-layout {
        display: flex;
        height: 100vh;
        gap: 0;
      }
      .snippet-panel {
        width: 320px;
        flex-shrink: 0;
        padding: 16px;
        display: flex;
        flex-direction: column;
        gap: 8px;
        overflow-y: auto;
        border-right: 1px solid #ddd;
        background: #fafafa;
      }
      .snippet-btn {
        padding: 10px 14px;
        border: 1px solid #ccc;
        border-radius: 6px;
        background: #fff;
        font-family: "JetBrains Mono", "Fira Code", monospace;
        font-size: 13px;
        color: #333;
        cursor: pointer;
        text-align: left;
        transition: all 0.2s;
        line-height: 1.4;
      }
      .snippet-btn:hover {
        border-color: #8083ff;
        background: #f0f0ff;
      }
      .snippet-btn.active {
        border-color: #8083ff;
        background: #e8e8ff;
        color: #571bc1;
        font-weight: 600;
      }
      .preview-panel {
        flex: 1;
        padding: 16px;
        display: flex;
        align-items: center;
        justify-content: center;
        background: #fff;
        overflow: auto;
      }
      .preview-panel > section,
      .preview-panel > div:not(.snippet-panel):not(.demo-layout) {
        flex: 1;
        width: 100%;
        min-height: 0;
        height: 100%;
        display: flex;
        align-items: center;
        justify-content: center;
      }
      .default-example {
        border: 1px solid #c5c5c5;
        width: auto;
        max-height: 300px;
        display: flex;
      }

      .default-example > div {
        background-color: rgba(0, 0, 255, 0.2);
        border: 3px solid blue;
        margin: 10px;
        flex-grow: 1;
        flex-shrink: 1;
        flex-basis: 0;
      }

      #example-element {
        background-color: rgba(255, 0, 200, 0.2);
        border: 3px solid rebeccapurple;
      }
    </style>
  </head>
  <body>
    <div class="demo-layout">
      <div class="snippet-panel">
        <button class="snippet-btn active" data-index="0">flex: 1;</button>
        <button class="snippet-btn" data-index="1">flex: 2;</button>
        <button class="snippet-btn" data-index="2">flex: 1 30px;</button>
        <button class="snippet-btn" data-index="3">flex: 1 1 100px;</button>
      </div>
      <div class="preview-panel">
        <section class="default-example" id="default-example">
          <div class="transition-all" id="example-element">Change me</div>
          <div>flex: 1</div>
          <div>flex: 1</div>
        </section>
      </div>
    </div>
    <script>
      const snippets = [
        `#example-element {
  flex: 1;
}`,
        `#example-element {
  flex: 2;
}`,
        `#example-element {
  flex: 1 30px;
}`,
        `#example-element {
  flex: 1 1 100px;
}`,
      ];

      let styleEl = document.createElement("style");
      document.head.appendChild(styleEl);

      function applySnippet(index) {
        styleEl.textContent = snippets[index];
        document.querySelectorAll(".snippet-btn").forEach((btn, i) => {
          btn.classList.toggle("active", i === index);
        });
      }

      document.querySelectorAll(".snippet-btn").forEach((btn) => {
        btn.addEventListener("click", () => applySnippet(parseInt(btn.dataset.index)));
      });

      applySnippet(0);
    </script>
  </body>
</html>

order

order 控制项目的排列顺序，默认值为 0。值越小越靠前，可以为负数。order 仅改变视觉顺序，不改变 DOM 顺序。

html

<div class="container">
  <div class="item" style="order: 3">A（视觉排第三）</div>
  <div class="item" style="order: 1">B（视觉排第一）</div>
  <div class="item" style="order: 2">C（视觉排第二）</div>
</div>

css

.container {
  display: flex;
}

.item {
  padding: 10px 20px;
  background: #e3f2fd;
  border: 1px solid #1976d2;
}

响应式排序示例：

css

/* 移动端：侧边栏在下方 */
.sidebar { order: 2; }
.main { order: 1; }

/* 桌面端：侧边栏在左侧 */
@media (min-width: 768px) {
  .sidebar { order: 1; }
  .main { order: 2; }
}

可访问性警告：order 仅改变视觉呈现顺序，屏幕阅读器仍按 DOM 顺序读取。如果视觉顺序与语义顺序不一致，可能对使用辅助技术的用户造成困惑。应优先通过调整 DOM 结构来控制顺序，仅在必要时使用 order。

order 交互演示（MDN）

设置 flex/grid 项目的排列顺序（数值越小越靠前）。

html

<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>order 属性演示 - MDN 示例</title>
    <meta name="description" content="演示order 属性演示效果。" />
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }
      body {
        font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
        min-height: 100vh;
        padding: 0;
      }
      .demo-layout {
        display: flex;
        height: 100vh;
        gap: 0;
      }
      .snippet-panel {
        width: 320px;
        flex-shrink: 0;
        padding: 16px;
        display: flex;
        flex-direction: column;
        gap: 8px;
        overflow-y: auto;
        border-right: 1px solid #ddd;
        background: #fafafa;
      }
      .snippet-btn {
        padding: 10px 14px;
        border: 1px solid #ccc;
        border-radius: 6px;
        background: #fff;
        font-family: "JetBrains Mono", "Fira Code", monospace;
        font-size: 13px;
        color: #333;
        cursor: pointer;
        text-align: left;
        transition: all 0.2s;
        line-height: 1.4;
      }
      .snippet-btn:hover {
        border-color: #8083ff;
        background: #f0f0ff;
      }
      .snippet-btn.active {
        border-color: #8083ff;
        background: #e8e8ff;
        color: #571bc1;
        font-weight: 600;
      }
      .preview-panel {
        flex: 1;
        padding: 16px;
        display: flex;
        align-items: center;
        justify-content: center;
        background: #fff;
        overflow: auto;
      }
      .preview-panel > section,
      .preview-panel > div:not(.snippet-panel):not(.demo-layout) {
        flex: 1;
        width: 100%;
        min-height: 0;
        height: 100%;
        display: flex;
        align-items: center;
        justify-content: center;
      }
      .default-example {
        max-height: 300px;
        display: flex;
        flex-flow: column;
      }

      .default-example > div {
        background-color: rgb(0 0 255 / 0.2);
        border: 3px solid blue;
        margin: 0.5rem;
        padding: 0.5rem;
        flex: 1;
      }

      #example-element {
        background-color: rgb(255 0 200 / 0.2);
        border: 3px solid rebeccapurple;
      }

      #example-element::after {
        content: attr(style);
        outline: 2px dashed;
        font-family: monospace;
      }
    </style>
  </head>
  <body>
    <div class="demo-layout">
      <div class="snippet-panel">
        <button class="snippet-btn active" data-index="0">order: 0;</button>
        <button class="snippet-btn" data-index="1">order: 3;</button>
        <button class="snippet-btn" data-index="2">order: -1;</button>
        <button class="snippet-btn" data-index="3">order: 2;</button>
      </div>
      <div class="preview-panel">
        <section class="default-example" id="default-example">
          <div class="transition-all" id="example-element">盒子 1：</div>
          <div style="order: 1">盒子 2：<code>order: 1;</code></div>
          <div style="order: 2">盒子 3：<code>order: 2;</code></div>
          <div style="order: 2">盒子 4：<code>order: 2;</code></div>
          <div style="order: 3">盒子 5：<code>order: 3;</code></div>
        </section>
      </div>
    </div>
    <script>
      const snippets = [
        `#example-element {
  order: 0;
}`,
        `#example-element {
  order: 3;
}`,
        `#example-element {
  order: -1;
}`,
        `#example-element {
  order: 2;
}`,
      ];

      let styleEl = document.createElement("style");
      document.head.appendChild(styleEl);

      function applySnippet(index) {
        styleEl.textContent = snippets[index];
        document.querySelectorAll(".snippet-btn").forEach((btn, i) => {
          btn.classList.toggle("active", i === index);
        });
      }

      document.querySelectorAll(".snippet-btn").forEach((btn) => {
        btn.addEventListener("click", () => applySnippet(parseInt(btn.dataset.index)));
      });

      applySnippet(0);
    </script>
  </body>
</html>

align-self

align-self 允许单个项目覆盖容器的 align-items，实现独立的交叉轴对齐。默认值为 auto，即继承容器的 align-items。

值	行为
`auto`	默认值，继承 `align-items`
`flex-start`	交叉轴起点对齐
`flex-end`	交叉轴终点对齐
`center`	交叉轴居中
`stretch`	拉伸填满
`baseline`	基线对齐

html

<div class="container">
  <div class="item">A</div>
  <div class="item self-end">B</div>
  <div class="item">C</div>
</div>

css

.container {
  display: flex;
  align-items: flex-start; /* 容器默认：顶部对齐 */
  height: 200px;
  border: 2px solid #1976d2;
}

.item {
  width: 80px;
  height: 40px;
  background: #e3f2fd;
  border: 1px solid #1976d2;
}

/* B 单独覆盖为底部对齐 */
.self-end {
  align-self: flex-end;
}

align-self 交互演示（MDN）

覆盖单个项目在交叉轴方向的对齐方式。

html

<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>align-self 属性演示 - MDN 示例</title>
    <meta name="description" content="演示align（self 属性演示）效果。" />
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }
      body {
        font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
        min-height: 100vh;
        padding: 0;
      }
      .demo-layout {
        display: flex;
        height: 100vh;
        gap: 0;
      }
      .snippet-panel {
        width: 320px;
        flex-shrink: 0;
        padding: 16px;
        display: flex;
        flex-direction: column;
        gap: 8px;
        overflow-y: auto;
        border-right: 1px solid #ddd;
        background: #fafafa;
      }
      .snippet-btn {
        padding: 10px 14px;
        border: 1px solid #ccc;
        border-radius: 6px;
        background: #fff;
        font-family: "JetBrains Mono", "Fira Code", monospace;
        font-size: 13px;
        color: #333;
        cursor: pointer;
        text-align: left;
        transition: all 0.2s;
        line-height: 1.4;
      }
      .snippet-btn:hover {
        border-color: #8083ff;
        background: #f0f0ff;
      }
      .snippet-btn.active {
        border-color: #8083ff;
        background: #e8e8ff;
        color: #571bc1;
        font-weight: 600;
      }
      .preview-panel {
        flex: 1;
        padding: 16px;
        display: flex;
        align-items: center;
        justify-content: center;
        background: #fff;
        overflow: auto;
      }
      .preview-panel > section,
      .preview-panel > div:not(.snippet-panel):not(.demo-layout) {
        flex: 1;
        width: 100%;
        min-height: 0;
        height: 100%;
        display: flex;
        align-items: center;
        justify-content: center;
      }
      .example-container {
        border: 1px solid #c5c5c5;
        display: grid;
        width: 200px;
        grid-template-columns: 1fr 1fr;
        grid-auto-rows: 80px;
        grid-gap: 10px;
      }

      .example-container > div {
        background-color: rgba(0, 0, 255, 0.2);
        border: 3px solid blue;
      }
    </style>
  </head>
  <body>
    <div class="demo-layout">
      <div class="snippet-panel">
        <button class="snippet-btn active" data-index="0">align-self: stretch;</button>
        <button class="snippet-btn" data-index="1">align-self: center;</button>
        <button class="snippet-btn" data-index="2">align-self: start;</button>
        <button class="snippet-btn" data-index="3">align-self: end;</button>
      </div>
      <div class="preview-panel">
        <section class="default-example" id="default-example">
          <div class="example-container">
            <div class="transition-all" id="example-element">One</div>
            <div>Two</div>
            <div>Three</div>
          </div>
        </section>
      </div>
    </div>
    <script>
      const snippets = [
        `#example-element {
  align-self: stretch;
}`,
        `#example-element {
  align-self: center;
}`,
        `#example-element {
  align-self: start;
}`,
        `#example-element {
  align-self: end;
}`,
      ];

      let styleEl = document.createElement("style");
      document.head.appendChild(styleEl);

      function applySnippet(index) {
        styleEl.textContent = snippets[index];
        document.querySelectorAll(".snippet-btn").forEach((btn, i) => {
          btn.classList.toggle("active", i === index);
        });
      }

      document.querySelectorAll(".snippet-btn").forEach((btn) => {
        btn.addEventListener("click", () => applySnippet(parseInt(btn.dataset.index)));
      });

      applySnippet(0);
    </script>
  </body>
</html>

Flex 格式化上下文

当元素的 display 设为 flex 或 inline-flex 时，会创建一个 Flex 格式化上下文（Flex Formatting Context，FFC）。FFC 是一种独立的布局环境，其内部元素的布局行为与普通流（Block Formatting Context / Inline Formatting Context）截然不同。

FFC 的核心特征：

脱离普通流：Flex 项目不再参与块级格式化或行内格式化，而是按照 Flex 算法排列
float 失效：Flex 项目上的 float 和 clear 无效，不会产生浮动效果
vertical-align 失效：Flex 项目上的 vertical-align 无效，对齐由 align-items / align-self 控制
外边距不折叠：Flex 项目之间的垂直外边距不会折叠，这与 BFC 中相邻块级元素的行为不同
匿名项目：容器内直接出现的文本节点会被自动包装为匿名 Flex 项目，参与布局计算
绝对定位子元素：设置 position: absolute 的 Flex 项目脱离 FFC，不参与伸缩与对齐，但仍以最近的定位祖先为参考

FFC 与 BFC 的关系：Flex 容器本身同时也是一个 BFC，这意味着它包含内部浮动、阻止外边距折叠、不被浮动元素覆盖。但容器内部的布局由 FFC 规则主导，而非 BFC 规则。

自动边距

在 Flex 布局中，项目上设置 margin: auto（或 margin-left: auto、margin-top: auto 等）会产生特殊行为：自动边距会吸收该方向上的所有剩余空间。这是 Flex 布局独有的特性，在普通流中不存在。

自动边距的优先级高于对齐属性：当项目在某个轴方向上设置了 auto 边距时，容器上对应的 justify-content 或 align-items 将对该项目失效，因为剩余空间已被自动边距吸收。

html

<div class="container">
  <div class="item logo">Logo</div>
  <div class="item nav">导航</div>
  <div class="item user">用户</div>
</div>

css

.container {
  display: flex;
  align-items: center;
  height: 60px;
  padding: 0 20px;
  border: 2px solid #1976d2;
}

.item {
  padding: 8px 16px;
  background: #e3f2fd;
  border: 1px solid #1976d2;
}

/* 导航区域推到右侧 */
.nav {
  margin-left: auto;
}

自动边距的常见用法：

场景	实现	效果
最后一项靠右	`margin-left: auto`	右侧项目被推到终点
水平垂直居中	`margin: auto`	项目在两轴方向都居中
分隔两组项目	第一组末尾设 `margin-right: auto`	两组分别靠左和靠右

css

/* 水平垂直居中：利用自动边距 */
.center-container {
  display: flex;
  /* 不需要 justify-content 和 align-items */
  width: 300px;
  height: 200px;
  border: 2px solid #1976d2;
}

.center-container .item {
  margin: auto; /* 四个方向自动边距，吸收所有剩余空间 */
}

自动边距与 gap 的交互：gap 定义的间距是固定的，不会被自动边距覆盖。自动边距吸收的是扣除 gap 之后的剩余空间。

代码示例

以下综合示例展示 Flex 布局常见模式的实现方式，更多实战案例参见 3-Flex经典布局。

水平垂直居中

html

<div class="center-box">
  <div class="content">居中内容</div>
</div>

css

.center-box {
  display: flex;
  justify-content: center; /* 主轴居中 */
  align-items: center;     /* 交叉轴居中 */
  width: 100%;
  height: 300px;
  border: 2px solid #1976d2;
}

.content {
  padding: 20px 40px;
  background: #e3f2fd;
}

粘性页脚

html

<div class="page">
  <header>头部</header>
  <main>主内容区</main>
  <footer>页脚</footer>
</div>

css

.page {
  display: flex;
  flex-direction: column; /* 垂直排列 */
  min-height: 100vh;
  margin: 0;
}

header, footer {
  flex: none; /* 固定高度，不伸缩 */
}

main {
  flex: 1; /* 占据剩余空间，将页脚推到底部 */
}

导航栏布局

html

<nav class="navbar">
  <div class="logo">Logo</div>
  <ul class="nav-links">
    <li><a href="#">首页</a></li>
    <li><a href="#">产品</a></li>
  </ul>
  <button class="login-btn">登录</button>
</nav>

css

.navbar {
  display: flex;
  align-items: center;
  padding: 0 20px;
  height: 60px;
  background: #333;
  color: white;
}

.logo {
  flex: none;
  font-size: 20px;
  font-weight: bold;
}

/* 导航链接推到右侧 */
.nav-links {
  display: flex;
  gap: 20px;
  margin-left: auto; /* 自动边距：吸收剩余空间，将后续元素推到右边 */
  list-style: none;
  padding: 0;
}

.login-btn {
  flex: none;
  margin-left: 20px;
}

Flex 布局属性体系总览

图表渲染中…

浏览器兼容性

Flexbox 已获得所有现代浏览器的全面支持，无需添加前缀即可安全使用。

特性	Chrome	Firefox	Safari	Edge	移动端
`display: flex`	29+	28+	9+	12+	iOS 9+
`flex-wrap`	29+	28+	9+	12+	iOS 9+
`gap`	84+	63+	14.1+	84+	iOS 14.5+
`flex` 简写	29+	28+	9+	12+	iOS 9+

关于 gap：gap 在 Flex 布局中的支持时间较晚（2020 年后），如果需要兼容旧版浏览器，仍需使用 margin 方案。gap 在 Grid 布局中的支持更早。

关于 IE：IE 11 对 Flexbox 有部分支持，但存在多个已知 Bug（如 flex: 1 的 flex-basis 计算错误、min-height 失效等）。如需支持 IE 11，建议查阅 Flexbugs 了解已知问题及规避方案。

最佳实践

优先使用 flex 简写：单独设置 flex-grow、flex-shrink、flex-basis 容易遗漏，简写更简洁且不易出错
优先使用 gap 替代 margin：gap 不会在容器边缘产生多余间距，代码更简洁
固定宽度使用 flex: none：侧边栏、图标等固定尺寸元素应设 flex: none，避免被意外伸缩
等分空间使用 flex: 1：flex: 1 的 flex-basis: 0% 确保严格等分，不受内容宽度影响
处理文本溢出时加 min-width: 0：Flex 项目默认 min-width: auto，不会缩小到内容宽度以下，需手动解除
避免过度嵌套：每层 Flex 容器都执行完整的布局算法，嵌套越深性能开销越大
order 仅用于视觉调整：不要用 order 改变语义顺序，应优先调整 DOM 结构
自动边距优先于对齐属性：利用 margin-left: auto 可以优雅地实现"推到右侧"等效果

常见问题

flex: 1 和 flex: auto 有什么区别？

核心区别在于 flex-basis：flex: 1 展开为 1 1 0%，flex: auto 展开为 1 1 auto。0% 意味着初始尺寸为 0，所有空间按 flex-grow 比例分配，实现严格等分；auto 意味着先保留内容固有尺寸，再分配剩余空间。需要严格等分时用 flex: 1，需要保留内容最小宽度时用 flex: auto。

为什么设置了 flex: 1 但项目没有等宽？

最常见的原因是 min-width: auto。Flex 项目默认不会缩小到内容宽度以下，长文本或不可换行内容会撑开项目。解决方案是添加 min-width: 0，并配合 overflow: hidden 和 text-overflow: ellipsis 处理溢出内容。

为什么 justify-content 不生效？

justify-content 仅在有剩余空间时生效。如果项目设置了 flex-grow 占满了主轴，或项目总宽度已超出容器，则没有剩余空间可供分配，对齐效果不可见。确保项目不会占满主轴，或使用 flex: none 保持项目原始尺寸。

align-content 为什么不生效？

align-content 需要同时满足两个条件：① flex-wrap: wrap（允许换行）；② 实际存在多行。如果只有一行，align-content 无效。此外，容器需要有足够的高度才能看到效果。

gap 和 margin 有什么区别？

gap 只作用于项目之间，不会在容器边缘产生间距，无需处理首尾元素的特殊情况；margin 会在所有方向生效，需要用 :last-child 等选择器手动清除末尾间距。gap 由容器设置，margin 由项目设置。优先使用 gap。

布局实战案例

以下案例来自 Flex 速查笔记，演示 Flex 的各类布局应用。

骰子的布局

骰子的一面，最多可以放置9个点。

如果不加说明，本节的HTML模板一律如下

html

<div class="box">
  <span class="item"></span>
</div>

上面代码中，div元素（代表骰子的一个面）是Flex容器，span元素（代表一个点）是Flex项目。如果有多个项目，就要添加多个span元素，以此类推

单项目

html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }

      html, body {
        width: 100vw;
        height: 100vh;
      }

      body {
        display: flex;
        align-items: flex-start;
        justify-content: flex-start;
        flex-wrap: wrap;
        align-content: flex-start;
        background: linear-gradient(top, #222, #333);
      }

      [class$="face"] {
        margin: 16px;
        padding: 4px;
        background-color: #e7e7e7;
        width: 104px;
        height: 104px;
        object-fit: contain;

        box-shadow: inset 0 5px white,
          inset 0 -5px #bbb,
          inset 5px 0 #d7d7d7,
          inset -5px 0 #d7d7d7;
        border-radius: 10%;
      }

      .pip {
        display: block;
        width: 24px;
        height: 24px;
        color: white;
        border-radius: 50%;
        margin: 4px;
        background-color: #333;
        box-shadow: inset 0 3px #111, inset 0 -3px #555;
        display: flex;
        align-items: center;
        justify-content: center;
      }

      .first1-face {
        display: flex;
      }

      .first2-face {
        display: flex;
        justify-content: center;
      }

      .first3-face {
        display: flex;
        justify-content: flex-end;
      }

      .first4-face {
        display: flex;
        align-items: center;
      }

      .first5-face {
        display: flex;
        justify-content: center;
        align-items: center;
      }
      .first6-face {
        display: flex;
        justify-content: flex-end;
        align-items: center;
      }
      .first7-face {
        display: flex;
        justify-content: flex-start;
        align-items: flex-end;
      }
      .first8-face {
        display: flex;
        justify-content: center;
        align-items: flex-end;
      }
      .first9-face {
        display: flex;
        justify-content: flex-end;
        align-items: flex-end;
      }

    </style>
  </head>
  <body>
    <div class="first1-face">
      <span class="pip">1</span>
    </div>
    <div class="first2-face">
      <span class="pip">2</span>
    </div>
    <div class="first3-face">
      <span class="pip">3</span>
    </div>
    <div class="first4-face">
      <span class="pip">4</span>
    </div>
    <div class="first5-face">
      <span class="pip">5</span>
    </div>
    <div class="first6-face">
      <span class="pip">6</span>
    </div>
    <div class="first7-face">
      <span class="pip">7</span>
    </div>
    <div class="first8-face">
      <span class="pip">8</span>
    </div>
    <div class="first9-face">
      <span class="pip">9</span>
    </div>
  </body>
</html>

双项目

html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>two-dice</title>
    <style>
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        html, body {
            width: 100vw;
            height: 100vh;
        }

        body {
            display: flex;
            align-items: flex-start;
            justify-content: flex-start;
            flex-wrap: wrap;
            align-content: flex-start;
            background: linear-gradient(top, #222, #333);
        }

        [class$="face"] {
            margin: 16px;
            padding: 4px;
            background-color: #e7e7e7;
            width: 104px;
            height: 104px;
            object-fit: contain;

            box-shadow: inset 0 5px white,
            inset 0 -5px #bbb,
            inset 5px 0 #d7d7d7,
            inset -5px 0 #d7d7d7;
            border-radius: 10%;
        }

        .pip {
            width: 24px;
            height: 24px;
            color: white;
            border-radius: 50%;
            margin: 4px;
            background-color: #333;
            box-shadow: inset 0 3px #111, inset 0 -3px #555;
            display: flex;
            align-items: center;
            justify-content: center;
        }

        .second1-face {
            display: flex;
            justify-content: space-between;
        }

        .second2-face {
            display: flex;
            flex-direction: column;
            justify-content: space-between;
        }

        .second3-face {
            display: flex;
            flex-direction: column;
            justify-content: space-between;
            align-items: center;
        }

        .second4-face {
            display: flex;
            flex-direction: column;
            justify-content: space-between;
            align-items: flex-end;
        }

        .second5-face {
            display: flex;
        }

        .second5-face .pip:nth-of-type(2) {
            align-self: center;
        }

        .second6-face {
            display: flex;
            justify-content: space-between;
        }

        .second6-face .pip:nth-of-type(2) {
            align-self: flex-end;
        }

    </style>
</head>
<body>
<div class="second1-face">
    <span class="pip">1</span>
    <span class="pip">2</span>
</div>
<div class="second2-face">
    <span class="pip">1</span>
    <span class="pip">2</span>
</div>
<div class="second3-face">
    <span class="pip">1</span>
    <span class="pip">2</span>
</div>
<div class="second4-face">
    <span class="pip">1</span>
    <span class="pip">2</span>
</div>
<div class="second5-face">
    <span class="pip">1</span>
    <span class="pip">2</span>
</div>
<div class="second6-face">
    <span class="pip">1</span>
    <span class="pip">2</span>
</div>
</body>
</html>

三项目

css

.box {
  display: flex;
}

.item:nth-child(2) {
  align-self: center;
}

.item:nth-child(3) {
  align-self: flex-end;
}

四项目

css

.box {
  display: flex;
  flex-wrap: wrap;
  justify-content: flex-end;
  align-content: space-between;
}

HTML代码如下

css

<div class="box">
  <div class="column">
    <span class="item"></span>
    <span class="item"></span>
  </div>
  <div class="column">
    <span class="item"></span>
    <span class="item"></span>
  </div>
</div>

.box {
	display: flex;
	flex-wrap: wrap;
	align-content: space-between;
}

.column {
	flex-basis: 100%;
	display: flex;
	justify-content: space-between;
}

六项目

css

.box {
  display: flex;
  flex-wrap: wrap;
  align-content: space-between;
}

css

.box {
  display: flex;
  flex-direction: column;
  flex-wrap: wrap;
  align-content: space-between;
}

html

<div class="box">
  <div class="row">
    <span class="item"></span>
    <span class="item"></span>
    <span class="item"></span>
  </div>
  <div class="row">
    <span class="item"></span>
  </div>
  <div class="row">
    <span class="item"></span>
    <span class="item"></span>
  </div>
</div>

.box {
	display: flex;
	flex-wrap: wrap;
}

.row{
	flex-basis: 100%;
	display:flex;
}

.row:nth-child(2){
	justify-content: center;
}

.row:nth-child(3){
	justify-content: space-between;
}

九项目

css

.box {
  display: flex;
  flex-wrap: wrap;
}

网格布局

基本网格布局

最简单的网格布局，就是平均分布。在容器里面平均分配空间，跟上面的骰子布局很像，但是需要设置项目的自动缩放

html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
      .Grid {
        display: flex;
      }

      .Grid-cell {
        flex: 1;
        height: 30px;
        border: 1px solid red;
      }
    </style>
  </head>
  <body>
    <div class="Grid">
      <div class="Grid-cell">...</div>
      <div class="Grid-cell">...</div>
      <div class="Grid-cell">...</div>
    </div>
  </body>
</html>

百分比布局

某个网格的宽度为固定的百分比，其余网格平均分配剩余的空间

html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>百分比布局</title>
    <style>
        .Grid {
            display: flex;
        }

        .Grid-cell {
            flex: 1;
            border:1px solid red;
        }

        .Grid-cell.u-full {
            flex: 0 0 100%;
        }

        .Grid-cell.u-1of2 {
            flex: 0 0 50%;
        }

        .Grid-cell.u-1of3 {
            flex: 0 0 33.3333%;
        }

        .Grid-cell.u-1of4 {
            flex: 0 0 25%;
        }
    </style>
</head>
<body>
<div class="Grid">
    <div class="Grid-cell u-1of4">...</div>
    <div class="Grid-cell">...</div>
    <div class="Grid-cell u-1of3">...</div>
</div>
</body>
</html>

圣杯布局

圣杯布局（Holy Grail Layout）指的是一种最常见的网站布局。页面从上到下，分成三个部分：头部（header），躯干（body），尾部（footer）。其中躯干又水平分成三栏，从左到右为：导航、主栏、副栏。

html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>圣杯布局</title>
    <style>
        *{
            margin: 0;
        }
        .HolyGrail {
            display: flex;
            min-height: 100vh;
            flex-direction: column;
        }

        header,
        footer {
            border: 1px solid red;
            height: 100px!important;
        }

        .HolyGrail-body {
            display: flex;
            flex: 1;
        }

        .HolyGrail-content {
            flex: 1;
        }

        .HolyGrail-nav, .HolyGrail-ads {
            /* 两个边栏的宽度设为12em */
            flex: 0 0 12em;
            border: 1px solid blue;
        }

        /*如果是小屏幕，躯干的三栏自动变为垂直叠加*/
        @media (max-width: 768px) {
            .HolyGrail-body {
                flex-direction: column;
                flex: 1;
            }
            .HolyGrail-nav,
            .HolyGrail-ads,
            .HolyGrail-content {
                flex: auto;
            }
        }
    </style>
</head>
<body class="HolyGrail">
<header>#header</header>
<div class="HolyGrail-body">
    <nav class="HolyGrail-nav">nav</nav>
    <main class="HolyGrail-content">#content</main>
    <aside class="HolyGrail-ads">ads</aside>
</div>
<footer>#footer</footer>
</body>
</html>

输入框的布局

我们常常需要在输入框的前方添加提示，后方添加按钮

html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .InputAddOn {
            display: flex;
        }

        .InputAddOn-field {
            flex: 1;
        }
    </style>
</head>
<body>
<div class="InputAddOn">
    <span class="InputAddOn-item">...</span>
    <input class="InputAddOn-field">
    <button class="InputAddOn-item">...</button>
</div>
</body>
</html>

悬挂式布局

有时，主栏的左侧或右侧，需要添加一个图片栏。

html

<div class="Media">
  <img class="Media-figure" src="" alt="">
  <p class="Media-body">...</p>
</div>
.Media {
  display: flex;
  align-items: flex-start;
}

.Media-figure {
  margin-right: 1em;
}

.Media-body {
  flex: 1;
}

固定的底栏

有时，页面内容太少，无法占满一屏的高度，底栏就会抬高到页面的中间。这时可以采用Flex布局，让底栏总是出现在页面的底部。

html

<body class="Site">
  <header>...</header>
  <main class="Site-content">...</main>
  <footer>...</footer>
</body>
.Site {
  display: flex;
  min-height: 100vh;
  flex-direction: column;
}

.Site-content {
  flex: 1;
}

流式布局

每行的项目数固定，会自动分行。

css

.parent {
  width: 200px;
  height: 150px;
  background-color: black;
  display: flex;
  flex-flow: row wrap;
  align-content: flex-start;
}

.child {
  box-sizing: border-box;
  background-color: white;
  flex: 0 0 25%;
  height: 50px;
  border: 1px solid red;
}

总结

Flex 布局是 CSS 一维布局的核心方案，其设计哲学是 声明式空间分配——开发者只需描述期望的布局行为（扩展、收缩、对齐），浏览器自动完成计算。掌握 Flex 布局的关键在于理解以下核心模型：

容器-项目二层结构：容器属性控制排列和对齐，项目属性控制伸缩和覆盖
双轴模型：justify-content 作用于主轴，align-items 作用于交叉轴，方向随 flex-direction 变化
伸缩三属性：flex-grow（扩展）、flex-shrink（收缩）、flex-basis（初始尺寸），推荐使用 flex 简写
FFC 格式化上下文：Flex 容器创建独立的布局环境，float 和 vertical-align 失效，外边距不折叠
自动边距：margin: auto 吸收剩余空间，优先级高于对齐属性

关于 Flex 布局的计算原理和深度机制，参见 2-flex计算原理；关于经典布局模式的实战案例，参见 3-Flex经典布局。

参考资料

W3C CSS Flexible Box Layout Module Level 1 — Flexbox 规范原文
MDN CSS Flexbox 指南 — Mozilla 官方文档
CSS-Tricks: A Complete Guide to Flexbox — 最流行的 Flex 速查手册
Flexbugs — 浏览器 Flex 实现的已知 Bug 列表
Flexbox Froggy — 交互式 Flex 学习游戏