/*利用<i>来安放图标*/
     .input_icon{
         display: inline-block;
         height: 16px;
         width: 100px;
         color: #ccc;
         position: absolute;
         top: 30%;
         left: 100px;
         z-index: 2;
     }
     /*利用背景图片安放图标*/
     #icon{
         padding: 0 10px 0 35px;
         background-image: url(search.svg);
         background-repeat: no-repeat;
         background-position: 10px 7px;
     }

     /*统一必须的红色样式*/
     #must{color: red}