黑豆的博客 
<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    /* reset.css */
    html,
    body,
    div,
    span,
    applet,
    object,
    iframe,
    h1,
    h2,
    h3,
    h4,
    h5,
    h6,
    p,
    blockquote,
    pre,
    a,
    abbr,
    acronym,
    address,
    big,
    cite,
    code,
    del,
    dfn,
    em,
    img,
    ins,
    kbd,
    q,
    s,
    samp,
    small,
    strike,
    strong,
    sub,
    sup,
    tt,
    var,
    b,
    u,
    i,
    center,
    dl,
    dt,
    dd,
    ol,
    ul,
    li,
    fieldset,
    form,
    label,
    legend,
    table,
    caption,
    tbody,
    tfoot,
    thead,
    tr,
    th,
    td,
    article,
    aside,
    canvas,
    details,
    embed,
    figure,
    figcaption,
    footer,
    header,
    hgroup,
    menu,
    nav,
    output,
    ruby,
    section,
    summary,
    time,
    mark,
    audio,
    video {
      margin: 0;
      padding: 0;
      border: 0;
      font-size: 100%;
      font: inherit;
      vertical-align: baseline;
    }

    /* HTML5 display-role reset for older browsers */
    article,
    aside,
    details,
    figcaption,
    figure,
    footer,
    header,
    hgroup,
    menu,
    nav,
    section {
      display: block;
    }

    body {
      line-height: 1;
    }

    ol,
    ul {
      list-style: none;
    }

    blockquote,
    q {
      quotes: none;
    }

    blockquote:before,
    blockquote:after,
    q:before,
    q:after {
      content: '';
      content: none;
    }

    table {
      border-collapse: collapse;
      border-spacing: 0;
    }
    /* 下面的样式是实现九宫格的核心样式 */
    body {
      padding: 50px;
    }

    .box {
      display: flex;
      flex-wrap: wrap;
      width: 300px;
    }

    li {
      width: 100px;
      text-align: center;
      line-height: 100px;
      box-sizing: border-box;
      border: 4px solid black;
      margin-left: -4px;
      margin-top: -4px;
    }

    li:nth-child(3n+1) {
      margin-left: 0;
    }

    li:hover {
      border-color: red;
      z-index: 2;
    }
  </style>
</head>

<body>
  <ul class="box">
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
    <li>7</li>
    <li>8</li>
    <li>9</li>
  </ul>
</body>

</html>

效果展示如下：

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