fieldset: a CSS block formatting context?

For the rendering section I suggest defining fieldset to be a block 
formatting context as defined in CSS 2.1. It is treated that way in 
Firefox, IE (8 and earlier), Opera and in Safari partially as well.

This means for example, that the fieldset's outer vertical margins do 
not collapse with its in-flow children's margins (Safari is the 
exception here, if it wasn't fixed in their trunk already).

~ds