IIRC, if you don't set
display to
block (I'm assuming you're using a
span, because you really shouldn't be using a
div inside a paragraph), most renderers will ignore the top margin. Depending on the font, you may also need to set the
line-height to at least
1.2, and you may want to set the box's
height property to avoid wasting too much space below the drop cap on whitespace.
There are lots of other fun surprises if you aren't careful. Take a look at the CSS section of the EPUB article on the wiki site. Most of those quirks are reader-specific bugs that I discovered while doing drop caps.