console 命令行工具 X clear
console
<!DOCTYPE html>
<html lang="zh-CN">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    body {
      font-size: 16px;
    }
    /* 设计图尺寸 640px  基准值 100px */
    div {
      width: 5rem;
      height: 2rem;
      font-size: .3rem;
      background-color: pink;
    }
  </style>
</head>
<body>
  <div>我是自适应的文字</div>
  <script>
    var design = 640;
    var base = 100;
    function responsive() {
      var width = window.innerWidth;
      // 所求font-size = 当前屏幕宽度 / 设计图 * 基准值
      var size = width / design * base;
      document.documentElement.style.fontSize = size + "px";
    }
    responsive();
    window.addEventListener("resize", responsive);
  </script>
</body>
</html>