<!DOCTYPE html>
<html>
<body>
<style>
#box{
width: 0;
height: 0;
border: 100px solid transparent;
border-bottom: 100px solid blue;
}
</style>
<div id="box"></div>
</body>
</html>
<!DOCTYPE html>
<html>
<body>
<style>
#box{
width: 0;
height: 0;
border: 100px solid transparent;
border-bottom: 100px solid blue;
}
</style>
<div id="box"></div>
</body>
</html>
![CSS的伪元素和伪类的区别[图]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)