How to position a div vertically and horizontally center

<style>

.parent_div{position:relative;float:left;width:500px;height:500px}

.inner_div{position:absolute;top:50%;left50%;transform: translate(-50%,-50%);-webkit-transform: translate(-50%,-50%);}

</style>

<div class=”parent_div”>

<div class=”inner_div”>

</div>

</div>