在一行中实现每个盒子间隔相等

达成效果：
在这里插入图片描述

1. 使用justify-content: space-evenly;

<!DOCTYPE html>
<html lang="zh-cn">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>flex evenly</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    .wrap {
      display: flex;
      justify-content: space-evenly;
      width: 700px;
      height: 400px;
      margin: 100px auto;
      background-color: #ccc;
    }
    
    .item {
      width: 140px;
      height: 120px;
      background-color: #368;
    }
  </style>
</head>

<body>
  <div class="wrap">
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
  </div>
</body>
</html>

缺点：苹果的兼容性不好，推荐以下方法

2. space-between配合before+after实现space-evenly效果

<!DOCTYPE html>
<html lang="zh-cn">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>flex evenly兼容</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    .wrap {
      display: flex;
      justify-content: space-between;
      width: 600px;
      height: 400px;
      margin: 100px auto;
      background-color: #ccc;
    }

    .wrap:before,
    .wrap:after {
     /* 用空内容来占位达到效果 */
      content: ''; 
    }

    .item {
      width: 140px;
      height: 120px;
      background-color: #368;
    }
  </style>
</head>

<body>
  <div class="wrap">
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
  </div>
</body>
</html>