下面我是通过class获取元素
<!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>Document</title><style>.bb {background: pink;}</style></head><body><div class="aa"><h1>hahah</h1></div><script>window.onload = function() {//通过类名获取元素var a = document.getElementsByClassName('aa')[0]//给元素设置样式a.style.backgroundColor = 'pink'}</script></body></html>
//后面不带索引值控制台出现报错var a = document.getElementsByClassName('aa')
