On http://test.csswg.org/suites/css2.1/20100917/html4/padding-bottom-080.htm I see a black box with a 1px red border at the bottom. The red border disappears at some non-default zoom levels. Safari and Chrome handle the test fine. (Note, you probably need the Ahem font installed in order to test this.) Mozilla/5.0 (Macintosh; Intel Mac OS X 10.5; rv:2.0b7pre) Gecko/20100924 Firefox/4.0b7pre <style type="text/css"> div { font: 12pt ahem; height: 0; } #div1 { border-top: 1ex solid red; padding-bottom: 1ex; } #div2 { border-bottom: 1ex solid black; margin-top: -2ex; } </style> <p>Test passes if there is no red visible on the page.</p> <div id="div1"></div> <div id="div2"></div> This is probably similar, if not the same, as bug 224753.

We round borders (but not paddings or margins) to device pixels, no? If so, I think this is either invalid or that decision is a css spec violation. Or both. ;)

The CSS spec doesn't define rounding behavior, so this is invalid.

On the other hand, shouldn't the Ahem font have an ex that's an integer number of px or something at 16px font size? It looks like ex ends up as 12.8px, which is what causes this problem....

An ex-height of 0.8 times the font size matches my memory of what Ahem is supposed to have. That matches your numbers and is not an integer.

Ah, ok. Perhaps the test should use a 20px font size?

This bug report was invalid and the test has been corrected accordingly (changed from 'font: 12pt ahem' to 'font: 20px/1 Ahem') 12 months ago: http://hg.csswg.org/test/rev/f75282aa9656 http://test.csswg.org/suites/css2.1/20110323/html4/padding-bottom-080.htm Resolving as INVALID