Question

I mean by not using any other tags…just one tag and the CSS for it.

So

http://jsfiddle.net/EqTsu/

<style>
  #test{
      width: 100px;
      height: 100px;
      text-align: center;
      border: solid 1px #ff0000;
    }
</style>

<div id='test'>foo</div>

needs what to center vertically?

Per Answers Below

It needs

  display: table-cell;
  vertical-align: middle;

Answer 1

There’s a sort of hack-ish work-around where you give the <div> the display: table-cell; property and then the vertical-align: middle; property, yes.

So the complete CSS would be:

#test{
  display: table-cell;
  vertical-align: middle;
  width: 100px;
  height: 100px;
  text-align: center;
  border: solid 1px #ff0000;
}

Also, external stylesheets are your friends.