When writing CSS most people use short hand because it saves space and thought.
For instance below says make a white backgorund with a horizontally repeating background image body_bgrd.png
background: #FFFFFF url(../images/body_bgrd.png) 0 0 repeat-x;
That works fine for your site's CSS but paste that into your HTML Mail template and your in for a surprise. :)
Instead you have to write the CSS out in long hand.