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: 600px;
height: 300px;
border: 1px solid #000;
margin: 100px auto;
/* background: linear-gradient(to right,#ff0000,#00ff00,#0000ff); */
/* background: linear-gradient(180deg,#ff0000,#00ff00,#0000ff); */
background: radial-gradient(circle,#ff0000,#00ff00,#0000ff);
}
</style>
</head>
<body>
<!--
径向渐变：
1，渐变形状，默认为椭圆形：ellipse
设置圆形：circle
2，渐变颜色：至少两个
径向渐变语法：
background: radial-gradient(渐变形状,开始颜色,过渡色1,过渡色2,......,终止颜色);
-->
<div class="box"></div>
</body>
</html>
