vertical centreing

Does anyone know why top: 50% is not working in this case? and my content which should be vertically centred is disappearing off the top of the screen?

html, body { height:100%, width: 100%; }
#holding-container { position: relative; height: auto; }
#holding {
/*   margin:0 auto;*/
    width:600px;
    height:700px;
    position:absolute;
    left:50%;
    top:50%;
    margin:-350px 0 0 -300px;
}

<div id="holding-container">
<div id="holding" style="text-align: center;"><img class="medium-large alignnone wp-image-48" src="..." alt="" width="600" height="600" />
<a style="text-decoration: none;">...</a></div>
</div>