Many HTML5 online resources are an improvement on the generally hit-and-miss approach to HTML that have characterized web development coverage in the past. The HTML5 specification is (for the most part) fairly clear and well understood, so long as the reader has the understanding that aspects of the spec will continue to change.

For learning HTML5 I would suggest sticking to sites that have an established track record of evangelizing the new language: they tend to demonstrate a thorough understanding of the specification and have adapted to changes. "Johnny come lately" sites tend to focus on one or two "cool features", rather than gaining the whole picture, and have articles that are preserved in digital amber, without regard to changes. Some suggestions for quality online sources would include:

"Dive Into HTML5" was one of the first HTML5 evangelist sites, and is still one of the best, evidenced by the fact that the text content of the site was used as the basis for a well-regarded O'Reilly book that I have suggested as a reference in the past.

HTML5 Doctor takes a different approach, "diagnosing" questions related to the spec. Also has a very useful HTML5 element index, with brief articles on each.

HTML5Rocks is another excellent site that keeps itself updated with articles. It also has a useful code sandbox playground, studio and extensive tutorials, which can be listed by feature type (including multimedia, storage, file access and mobile development).

Of course the canonical references continue to be the W3C HTML5 and WHATWG sites, but those may be a little too technical for most.

If you have other suggestions for online HTML5 resources, please feel free to suggest them in the comment section below!