table rowsep/colsep ignored in HTML but not FO

I have a table which looks fine when I generate FO - it has rowseps
and colseps exactly where I told it to put them, according to the
<table> and <row> attributes.

For some reason, in HTML, I see borders around every cell, regardless
of what my rowsep/colset attribs are. Any idea why that would be? Is
it some parameter I need to set related to CSS?