媒体查询实例（手机为先） -

<!DOCTYPE html>
<html lang="zh-CN">
<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>媒体查询</title>
</head>
<body>
  <button class="btn small">btn1</button>
  <button class="btn middle">btn2</button>
  <button class="btn large">btn3</button>
</body>
</html>
<style>
  html {
    font-size: 0.625rem;
  }
  .btn {
    background-color: rebeccapurple;
    color: white;
    border: none;
    outline: auto;
  }
  .btn:hover {
    cursor: pointer;
    opacity: 0.7;
    transition: 0.4s;
  }
  .btn.small {
    font-size: 1.4rem;
  }
  .btn.middle {
    font-size: 1.8rem;
  }
  .btn.large {
    font-size: 2.2rem;
  }
  @media (max-width: 375px) {
    html{
      font-size: 14px;
    }
  }
   @media (min-width: 375px) and (max-width: 600px) {
    html{
      font-size: 17px;
    }
  }
   @media (min-width: 600px) {
    html{
      font-size: 20px;
    }
  }
</style>