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<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
body{
background:blue;
}
div{
width:0px;
height: 0px;
border-top:50px solid transparent;
border-right:50px solid red;
border-bottom:50px solid transparent;
border-left:50px solid transparent;
}
</style>
</head>
<body>
<div></div>
</body>
</html>
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原文：https://www.cnblogs.com/XFlyaa/p/10547339.html