一、概念

把视口的width、height平均分成100份。1vw就是一份的width，同理1vh就是一份的height。

二、实例演示

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>vw、vh Demo</title>
  <style>
    * {
      padding:0;
      margin:0;
    }
    .red {
      background: red;
    }
    .green {
      background: green;
    }
    .blue {
      background: blue;
    }
    .orange {
      background: orange;
    }
    .vw25 {
      width:25vw;
    }
    .vw50 {
      width: 50vw;
    }
    .vw100 {
      width: 100vw;
    }
    .vh25 {
      height: 25vh;
    }
    .vh50 {
      height: 50vh;
    }
    .vh100 {
      height: 100vh;
    }
    .fl {
      float: left;
    }
    .clearfix {
      clear: both;
    }
    .clearfix::after {
      clear: both;
      content:'';
      display: block;
    }
    .flexBlock {
      display: flex;
      justify-content: center;
      align-items: center;
      color:#fff;
      font-size: 16px;
    }
  </style>
</head>
<body>
  <div class="clearfix">
    <div class="vw25 vh50 red fl flexBlock">
      25vw   50vh 效果图
    </div>
    <div class="vw25 vh50 green fl flexBlock">
      25vw   50vh 效果图
    </div>
    <div class="vw25 vh50 red fl flexBlock">
      25vw   50vh 效果图
    </div>
    <div class="vw25 vh50 green fl flexBlock">
      25vw   50vh 效果图
    </div>
  </div>
  <div class="clearfix">
    <div class="vw50 vh25 fl blue flexBlock">
      50vw  25vh  效果图
    </div>
    <div class="vw50 vh25 fl orange flexBlock">
      50vw  25vh  效果图
    </div>
  </div>
  <div class="vw100 vh25 red flexBlock">
    100vw  25vh  效果图
  </div>
  <div class="clearfix">
    <div class="vw50 vh100 fl blue flexBlock">
      50vw  100vh  效果图
    </div>
    <div class="vw50 vh100 fl green flexBlock">
      50vw  100vh  效果图
    </div>
  </div>
</body>
</html>

效果如下: