Theora, Dirac, Ogg, and Vorbis are standards, even if they're not referenced by HTML5. Acid tests have, at times, tested things that were not W3C standards, like CSS 2.1 and CSS 2.0 features that had been dropped from CSS 2.1. They're always tests of those features that the developers think should be available, not of a particular HTML version and its references. HTML4 doesn't specify that ECMAScript/javascipt is supported at all, and the latest revision of HTML4 predates DOM2 and CSS 2.1 entirely. Acid2 tests transparency in PNG, which (like PNG in general) isn't required by W3C standards. So there's no reason Acid4 couldn't simply state that it tests Ogg-related formats as one of the items that they think a browser should support correctly.