我的span为什么不能设置absolute绝对定们呢？

Question

           <ul class="header_login">              <li>下载</li>              <li>马上登录</li>              <li>注册</li>          </ul>          <div class="header_search_wrapper">          <form>            <span id="search_tab">                 <a href="#">商品</a>                 <i class="icon_1"></i>                 <span class="header_search_sub">                 <a href="#">商品</a>                 <a href="#">店铺</a>                 </span>             </span>             <span>            <input type="text" required="required" class="search_input" placeholder="请输入关键词……">            <input type="submit" value="搜索">            </span>          </form>          </div> .header_search_wrapper { /* 包装器的相对定位*/ position:relative; float: right; height: 45px; width: 425px; line-height: 45px; margin: 13px; border: 2px solid #7FC7A0; background-color: #474645; background-image: -webkit-linear-gradient(270deg,rgba(255,255,255,1.00) 0%,rgba(97,96,95,1.00) 20.73%,rgba(71,70,69,1.00) 100%); background-image: -moz-linear-gradient(270deg,rgba(255,255,255,1.00) 0%,rgba(97,96,95,1.00) 20.73%,rgba(71,70,69,1.00) 100%); background-image: -o-linear-gradient(270deg,rgba(255,255,255,1.00) 0%,rgba(97,96,95,1.00) 20.73%,rgba(71,70,69,1.00) 100%); background-image: linear-gradient(180deg,rgba(255,255,255,1.00) 0%,rgba(97,96,95,1.00) 20.73%,rgba(71,70,69,1.00) 100%);}/* span 的绝对定位*/.header_search_sub {width: 52px; display:block; position:absolute;}.header_search_sub a {background-color:#FbFbFb; display:block; color:#7B7E7D;  border:2px #81CF82 solid;}.search_input {height:30px; width:300px;}.header_login {float:right; line-height:70px; list-style:none;}.header_login li {float:left; width:75px; color:#747878;}.icon_1 {width:15px; height:12px; background:url(../images/i_1.jpg) left no-repeat; display:inline-block;}/* span 的绝对定位*/.header_search_sub {width: 52px; display:block; position:absolute; left:-1px;}.header_search_sub a {background-color:#FbFbFb; display:block; color:#7B7E7D;  border:2px #81CF82 solid;}.search_input {height:30px; width:300px;}.header_login {float:right; line-height:70px; list-style:none;}.header_login li {float:left; width:75px; color:#747878;}.icon_1 {width:15px; height:12px; background:url(../images/i_1.jpg) left no-repeat; display:inline-block;}

Answer 1

知道了，原来是我自己对三个定位的方式没有认真理解。