non funziona, dove sbaglio?

<script type="text/javascript">
function visible(x) {
x.style.visibility = 'visible';

}

function novisible(x) {
x.style.visibility = 'hidden';

}
</script>

<div id="box3" style="left:10px;top:10px;z-index: 5;position:absolute; width:300px;height:300px; background-color:green; visibility:hidden" onmouseover="visible(this);" onmouseout="novisible(this);">