rem 使用

 1 html{
 2     font-size:12px;
 3 }
 4 .btn {
 5      6rem;
 6     height: 3rem;
 7     line-height: 3rem;
 8     font-size: 2rem;
 9     display: inline-block;
10     background: #06c;
11     color: #fff;
12     border-radius: .5rem;
13     text-decoration: none;
14     text-align: center;    
15 }
16 .p{font-size: 2rem;}
17 </style>
18 <script>
19 alert("ddd");
20 document.getElementsByTagName('html')[0].style.fontSize = "20px"; 
21 alert(document.getElementsByTagName('html')[0].style.fontSize);
22 
23 //1.6rem*window.innerWidth / 320 + 'px';
24 </script>
25 </header>
26 <body>
27 
28 <div>
29 qwertyuiop
30 <p class="p">yyyyyy</p>
31 <button class="btn"> click </button>
32 </div>
33 
34 </body>
35 </html>