<div></div>

div {
    width: 100px;
    height: 100px;
    background: red;
    -webkit-transition: width 2s; /* Safari */
    transition: width 2s;/* this determines how long it takes to expand*/
}

/* For Safari 3.1 to 6.0 */
div {-webkit-transition-timing-function: linear;}


/* Standard syntax */
div {transition-timing-function: linear;}

div:hover {
    width: 300px;/*final width
}