怎么实现登录切换

   <!-- 登录前 -->
<div class="header">
    <div class="hd-cont before-log">
        <div class="hd-l"><a class="zm" href="">放到桌面</a>
            <a class="sy" href="">设为首页</a>
            <a class="sc" rel='sidebar' href="">加入收藏</a>
        </div>
        <ul class="hd-r">
            <li class="dl"><a href="user.html">登录</a></li>
            <li> | </li>
            <li class="zc"><a href="reg.html">注册</a></li>
        </ul>
    </div>
</div>   <!-- 登录后 -->
<div class="header">
    <div class="hd-cont after-log">
        <div class="hd-l"><a class="zm" href="">放到桌面</a><a class="sy" href="javascript:void(setHomepage())">设为首页</a><a class="sc"  rel='sidebar' href="javascript:void(addFavorite())">加入收藏</a></div>
        <ul class="hd-r">
            <li class="nail"><span></span></li>
            <li class="user-center"><a href="user-conter.html">l88888</a></li>
            <li class="logout"><a href="">[注销]</a></li>
        </ul>
    </div>
</div>