border实现：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style type="text/css">
.left{
width:0px;
height:0px;
border-top:50px solid rgba(0,0,0,0);
border-right:50px solid rgba(0,0,0,0);
border-bottom:50px solid blue;
border-left:50px solid rgba(0,0,0,0);
}
</style>
</head>
<body>
<div class="left"></div>
</div>
</body>
</html>