positioned elements: center

I don't know if this subject has been broached before...

Using absolute positioning to center an element is not as simple as  
it should be. I can position the element by setting its top, bottom,  
right, and left. But if I set "left:50%; top:50%;", but to center the  
whole element (instead of just the top left corner) I have to know  
its height and width, divide them in half, and use them as negative  
margin. It would be much simpler if I could set "center-x:50%; center- 
y:50%" instead of top, left, and negative margin.

I know there are a couple of other ways to get the items to center,  
but they are no simpler, or have other limitations. Being able to  
position items by center would be so much simpler.