简单来说%是浏览器百分比单位，em是电脑屏幕的百分比单位
<!DOCTYPE html><html><head> <meta charset="utf-8"> <meta http-equiv="X-UA-Compatible" content="IE=edge"> <title></title> <link rel="stylesheet" rel="external nofollow" href=""></head><style type="text/css" media="screen"> *{ padding: 0px; margin: 0px; } .bigbox{ width: 100px; height: 100px; margin-left: 20%; background: red; float:left; } .smallbox{ width: 100px; height: 100px; margin-left:20em; background:yellow; } body{ background: #2FB0FF; }</style><body > <div class="bigbox"> </div> <div class="smallbox"> </div></body></html>