<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>测试左上角小三角</title>
<style>
#div{
height:100px;
border:solid 1px #333;
position: relative;
}
#div::before{
content: '';
position: absolute;
left: 0;
top: 0;
width:0;
height:0;
border:8px solid rgba(0,0,0,0);
border-left:8px solid red;
border-top:8px solid red;
}
</style>
</head>
<body>
<div id="div"></div>
</body>
</html>
效果是这样的：
搜狗截图20190802151949.jpg
