在html里面一旦遇到float，

就要在外面加上class=“fixclear”


<!DOCTYPE html>
 <html>
 <head lang="en">
     <meta charset="UTF-8">
     <title></title>
     <style>
         .k:hover{
             background-color: red;
         }
         .l:before{
             content: "999";
         }
         .left{
             float: left;
         }
         .item:after{
             content: ".";
             clear: both;
             display: block;
             visibility: hidden;
             height: 0px;
         }


     </style>
 </head>
 <body>
         <div class="k">normal</div>
         <div class="l">good</div>


         <div style="background-color: red" class="item">
             <div class="left" style="height: 100px;background-color: green">




                 2
             </div>


                <div class="left">1</div>
                 <!--<div style="clear:both;"></div>清除浮动的第一种方式-->




         </div>


 </body>
 </html>