making span tag cover entire width of div

I have the following structure for html:-

<div>
  <span>
    <a href="wherever">Wherever</a>
  </span>
</div>

If I want the span to cover the entire width of the div, how can I do that in chrome?

I tried in css using

span{
  background:#FFF;
  width:100%;
}

It works in firefox and ie but not in chrome.