Opera have started to publish their Opera Web Standards Curriculum, which is the first guide to authoring webpages that I've ever been happy to recommend.
It has two main drawbacks at present. First, it has some errors. Second, it doesn't cover everything it should.
Good news! Opera are listening to feedback and correcting errors (even ones regarding obscure points of SGML), and there is more content to come we've just seen the first wave so far.
Kudos to Opera.
Edit: The content has been migrated to a Wiki hosted by the W3C, and "Opera" has been dropped from the name. The link has been updated.