<div class="top">
    <div class="left">欢迎来到！</div>
    <div class="right">登录|注册</div>
</div>


.top .left{
    float: left;
}
.top .right{
    float: right;
}
.top:after{     //
    content:"";
    height:0px;
    width:0px;
    clear:both;
    display:block;
    overflow: hidden;
}