css

by Mike Lin

HTML

<div id="box">
<div class="a"></div>
</div>

CSS

#box{
        background-color:gray;
        height:20px;
        overflow:hidden;
    }
    .a{
        background-color:red;
        width:0;
        height:20px;
        margin: 0 auto;
        transition:width .8s;
        -webkit-transition:width .8s;
    }
    #box:hover .a{
        width:100%;
        transition:width .8s;
        -webkit-transition:width .8s;
    }