Possible Duplicate:
Why don't margin-top: auto and margin-bottom:auto work the same as their left and right counterparts?
What is the rationale behind margin-top: auto and margin-bottom: auto amounting to 0 margin?
Why doesn’t margin: auto auto; add vertical centering? Shouldn’t the auto keyword tell the browser to split the margins equally between the top and bottom? I was under the impression that was what it does for left and right margins.
Ultimately it’s a w3c spec –
It does seem like it would be handy.