因为在开发App时，为了自定义返回栏，返回箭头，一般都用图片，当图片不方便，最好用css样式实现。
逻辑：
画出一个正方形，让它旋转45度，只显示你需要的两个边即可
代码
<!DOCTYPE html>
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="viewport"
content="width=device-width,minimum-scale=1.0,maximum-scale=1.0,user-scalable=no,initial-scale=1">
<meta name="format-detection" content="telephone=no">
<meta name="apple-mobile-web-app-status-bar-style" content="white">
<meta name="apple-mobile-web-app-capable" content="no">
<title>Css arrow</title>
</head>
<style type="text/css">
#triangle-facing-right {
display: inline-block;
margin: 72px;
border-right: 24px solid;
border-bottom: 24px solid;
width: 120px;
height: 120px;
transform: rotate(-45deg);
}
#triangle-facing-left {
display: inline-block;
margin: 72px;
border-left: 24px solid;
border-bottom: 24px solid;
width: 120px;
height: 120px;
transform: rotate(45deg);
}
</style>
<body>
<div id="triangle-facing-right"></div>
<div id="triangle-facing-left"></div>
</body>
</html>
