*{margin:0;padding:0}
body{background:#ddd}
a{color:#fff; text-decoration:none;font-size:18px;display:block;width:120px;height:40px;line-height:40px;text-align:center;background:#30A7DB;margin:50px auto; position:relative;transition:.3s;-moz-transition:.3s;-webkit-transition:.3s;-o-transition:.3s;z-index:0}
a:before{ content:'';position:absolute;top:0;left:0;height:40px;width:0;background:#fff;transition:.3s;-moz-transition:.3s;-webkit-transition:.3s;-o-transition:.3s;z-index:-1}
a:hover{color:#30A7DB}
a:hover:before{width:120px;}