1.圆形与椭圆
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>opacity</title>
<style>
.circle {
border-radius: 50%;
width: 20px;
height: 20px;
background: red;
/* 宽度和高度相等，为圆；不相等为椭圆 */
}
</style>
</head>
<body>
<div>
<div class="circle"></div>
</div>
</html>
