Create an expandable button in width, with 3 background image in css (left, middle and right parts). All this keeping the html very simple.

expandable-button.css

.cbtn {
    position: relative;
    top: 0px;
    left: 9px; /* width of left part (:before) */
    overflow: visible; /* this make the :before and :after in position:absolute to appear */

    background: url(img/middle.png);
    height: 50px;

    padding: 0;
    border: 0;
    cursor: pointer;
}

.cbtn::-moz-focus-inner,
.cbtn:focus {
    border: 0;
}

.cbtn:before,
.cbtn:after {
    content: "";
    background: url(img/left.png);
    width: 9px;
    height: 50px;
    position: absolute;
    left: -9px; /* Same as width */
    top: 0px;
}

.cbtn:after {
    background: url(img/right.png);
    width: 9px;
    left: auto;
    right: -9px; /* Same as width */
}

.cbtn:hover,
.cbtn:hover:before,
.cbtn:hover:after {
    background-position: 0 -50px;
}

.cbtn:active,
.cbtn:active:before,
.cbtn:active:after {
    background-position: 0 -100px;
}

expandable-button.html

<button class="cbtn">Click me!!!</button>