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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>正方形</title>
<style>
.div{
width: 0px;
height: 0px;
border-top:50px solid red;
border-bottom:50px solid green;
border-left:50px solid yellow;
border-right:50px solid blue;
/*注意：四条边框的宽度必须相同！*/
}
</style>
</head>
<body>
<div class="div"></div>
</body>
</html>
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