注意先垂直方向，再水平方向
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>css</title>
<style type="text/css">
div{width:300px;height:100px;background:#f2f2f2;line-height:100px;text-align:center;
border-top:2px blue solid;
border-bottom:2px blue solid;
border-left:2px blue solid;
border-right:2px blue solid;
border-bottom-left-radius:10px 10px;
}
</style>
</head>
<body>
<div>我要好好学习</div>
</body>
</html>
返回值：