A leading Mozilla developer about the Acid 3:
http://shaver.off.net/diary/2008/03/27/the-missed-opportunity-of-acid-3/
Some examples:
Ians Acid 3, unlike its predecessors, is not about establishing a baseline of useful web capabilities. ...
We will fix standards compliance bugs; its what we do. But we wont fix them all with the same priority, and I hope that we wont prioritize Acid 3 fixes artificially highly, because I think that would be a disservice to web developers and users. Where Acid 3 happens to test something that we believe is important to fix, we will of course pursue it: surrogate pair handling or some of the selector bugs seem like good candidates. ....
Acid 3 could have had a tremendous positive effect on the web, representing the next target for the web platform, and helping developers prioritize work in such a way as to maximize the aggregate capabilities of the web. Instead, it feels like a puzzle game, and I can easily imagine the developers of the webs proprietary competitors chuckling about the hundreds of developer-hours that have gone into adding another way to iterate over nodes, or twiddling internal APIs to special case a testing font. I dont think its worthless, but I think it could have been a lot more, especially with someone as talented and terrifyingly detail-oriented as Ian at the helm.