<!DOCTYPE html>
<html lang="zh-cn">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
#dom2{
width:450px;
height: 450px;
background-color: black;
}
#as{
background-color: red;
}
</style>
</head>
<body>
<div id="dom2"></div>
<div id="as"></div>
<script>
var width=document.getElementById('dom2').offsetWidth;
var height=document.getElementById('dom2').offsetHeight;
var nh=height*16/9;
document.getElementById('as').style.width=width+"px";
document.getElementById('as').style.height=nh+"px";
</script>
</body>
</html>
