演示代码：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<link rel="stylesheet" type="text/css" href="reset.css"/>
<style type="text/css">
.box{
width: 1000px;
height: 200px;
border: 1px solid #000;
margin: 100px auto;
/* 从完全不透明红色开始,到完全透明红色结束 */
/* background: linear-gradient(to right,rgba(255,0,0,1),rgba(255,0,0,0)); */
/* background: linear-gradient(to right,rgba(255,0,0,1),rgba(255,0,0,0),rgba(0,0,255,0),rgba(0,0,255,1)); */
background: radial-gradient(circle,rgba(255,0,0,1),rgba(255,0,0,0));
}
</style>
</head>
<body>
<div class="box"></div>
</body>
</html>
