I'm not sure if I completely understand your question but I'll take a stab that you are talking about tables. There is a default number for cell spacing and padding with tables. If you don't specify them, it uses that number (which is 1 or 2 pixels or something). To eliminate that space, just add cellspacing="0" and cellpadding="0" within your table tag.
I know this isn't CSS, but it will still work. I haven't used CSS lately so I don't know how to do fix this with CSS.
