<!Doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>test</title>
<style>
*{margin:0;padding:0;}
body{background: green;}
#main{width:100%;
height:100%;
background: yellow;
margin-top:50px;
}
</style>
</head>
<body>
<div id="main">
</div>
</body>
</html>After adding the header declaration <!Doctype html>, the div cannot be displayed. After removing it, it can be displayed normally. The same situation occurs when tested in IE11 and chrome browsers. What is the problem?
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
You can see it by adding a border to main