效果图：
实例代码：
1 <!DOCTYPE html> 2 <html> 3 4 <head> 5 <meta charset="UTF-8"> 6 <title></title> 7 <style type="text/css"> 8 #btn { 9 display: block; 10 width: 100px; 11 height: 40px; 12 background-color: lightcoral; 13 text-align: center; 14 line-height: 40px; 15 position: relative; 16 color: #FFF; 17 font-size: 2em; 18 margin: 0 auto; 19 } 20 21 #btn::before { 22 content: ""; 23 width: 30px; 24 height: 30px; 25 border-radius: 30px; 26 background: #fff; 27 position: absolute; 28 display: block; 29 left: -15px; 30 top: 5px; 31 } 32 </style> 33 </head> 34 35 <body> 36 <div id="btn"></div> 37 </body> 38 39 </html>