When good tests go bad: Firefox on Acid(2)

Tags: , , , , , , , — Jeff @ 00:43

This is what the Acid2 test looks like in the very very super-duper-latest Firefox builds (slated for the version after 3.1, mind, not for 3.1):

Acid2 in bleeding-edge Mozilla, to be seen in the next Firefox release after 3.1; the chin is red whereas the reference image would have it yellow
Acid2 in bleeding-edge Mozilla, to be seen in the next Firefox release after 3.1

Bug, you say? No! The last test in row 13 tests that this CSS doesn’t apply:

.parser { background: red pink; }

Two colors for a background as valid CSS? Surely you jest!

Actually, we don’t. CSS3 says background-color takes two colors, one for normal use and one for use if a corresponding background-image doesn’t load. The red ends up getting used as the background color, and the yellow that would have been present if the property hadn’t applied no longer shows. CSS3 makes previously-invalid CSS valid, and that’s okay, because CSS error handling is explicitly designed to allow it.

So no, “failing” Acid2 for now isn’t fail, it’s WIN. Hopefully the test will be updated soon so that that particular rule is invalid again.

Incidentally, the specification for this changed recently, which I presume is why WebKit/Safari/Chrome et al. don’t fail. I don’t know whether they implement the fallback color process or not, presumably not given how they handle this testcase, which should show a green square if CSS3 background colors are implemented to latest spec, but in any case they won’t trigger it even tho they’ve implemented a bunch of other parts of the CSS3 backgrounds spec.

Update: As Dan notes in comments, the once-invalid syntax is now invalid again, as fallback color support has been removed from CSS3 partially over syntax concerns and partially over its lack of generalizability to other cases in CSS. Now that support for fallback colors has been removed from trunk, Acid2 in bleeding-edge Firefox now displays as it was intended to display.


  1. Interesting twist!

    Comment by David Naylor — 20.02.09 @ 07:34

  2. […] fazendo muito bem seu trabalho. Por exemplo, parece contraditório, mas recentemente a raposa deixou de passar no teste Acid2 por implementar uma característica do CSS3 em que pode-se carregar duas cores de fundo. A questão […]

    Pingback by PC World diz que o “Firefox está morto”; será? | BLOG.MACMAGAZINE — 28.03.09 @ 07:35

  3. That’s a really unintuitive syntax. The spec should change. Nobody’s going to be able to guess what ‘background: blue yellow’ means. IMHO.

    [I’m not sure I disagree. It still doesn’t change that, for now, it is acceptable to “fail” Acid2 so long as it’s for this specific reason, which was the primary reason for the post. Someone had to make the post so that not-CSS-hyper-expert nightly users could be pointed somewhere to explain the breakage, and when dbaron explained the history of his multiple-backgrounds commit a couple months back I thought it too hilarious for me not to talk about it. 🙂 ]

    Comment by Ian Hickson — 19.04.09 @ 16:14

  4. This issue has been “fixed” in bug 496721 by removing support for a fallback colour (due to CSS WG decisions).

    Comment by Dan — 17.06.09 @ 08:30

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