Acid3 was not updated when people decided to change the specifications. There are 3 things that are known to fail. Two of them are related to one change (see https://github.com/whatwg/dom/issues/319#issuecomment-296345... for details on that change). The third test is one where Acid3 mandates something that every browser did but was wrong per the specification, and it took several years for people to notice (see https://bugzilla.mozilla.org/show_bug.cgi?id=1311329 for some more details).

The Acid3 test on that website is officially unmaintained: https://lists.w3.org/Archives/Public/www-archive/2017Jul/000...