<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
* {margin: 0; padding: 0;}
.clearfix{zoom:1;/*为IE6，7的兼容性设置*/}
.clearfix:after {
content: '.';
display: block;
height: 0;
clear: both;
visibility: hidden;
}
ul li {
list-style: none;
float: left;
margin: 50px 0 50px 20px;
text-align: center;
}
li {
background: red;
}
h2 {
margin-top: 20px;
}
.circle1 {
width: 100px;
height: 50px;
border-radius: 50px 50px 0 0;
line-height: 50px;
}
.circle2 {
width: 50px;
height: 100px;
border-radius: 0 50px 50px 0;
line-height: 100px;
}
.circle3 {
width: 100px;
height: 50px;
border-radius: 0 0px 50px 50px;
line-height: 50px;
}
.circle4 {
width: 50px;
height: 100px;
border-radius: 50px 0 0 50px;
line-height: 100px;
}
.circle5 {
width: 100px;
height: 100px;
border-radius: 50px;
line-height: 100px;
}
.demo { /*左半圆*/
position:absolute; /*clip 属性剪裁绝对定位元素。也就是说，只有 position:absolute 的时候才是生效的。*/
width: 100px;
height: 100px;
border-radius: 50px;
/* line-height: 50px; */
clip: rect(0px 50px 100px 0px); /*唯一合法的形状值是：rect (top, right, bottom, left)*/
}
.right-circle { /*右半圆*/
left: 200px;
clip: rect(0px 100px 100px 50px); /*唯一合法的形状值是：rect (top, right, bottom, left)*/
}
</style>
</head>
<body>
<div>
<h2>用border-radius实现半圆</h2>
<ul class="clearfix">
<li class="circle1">上边圆</li>
<li class="circle2">左边圆</li>
<li class="circle3">下边圆</li>
<li class="circle4">左边圆</li>
<li class="circle5">全圆</li>
</ul>
<hr>
<h2>css3的clip 方法剪裁实现半圆</h2>
<p style="color: red;">clip 属性剪裁绝对定位元素。也就是说，只有 position:absolute 的时候才是生效的。唯一合法的形状值是：rect (top, right, bottom, left)</p>
<ul class="clearfix" style="position: relative;">
<li class="demo">左半圆</li>
<li class="demo right-circle">右半圆</li>
<li></li>
</ul>
</div>
</body>
</html>
效果图：
感兴趣的朋友可以去熟试下！！！