Here’s why, according to the WHATWG: “the term (HTML5) is now basically being used to mean anything Web-standards-related, so it’s time to move on!”
This may seem like an obscure technical debate, but it has real-world effects. Lack of agreement makes it harder to design Web sites and applications that work the same way on every platform. This undercuts one of the big promises of software as a service — the ability to use the same Web app (and its data) from any device with an Internet connection. Write once, run anywhere? More like write once, tweak endlessly.
The real way that Web “standards” get worked out is through real-world implementation. When enough Web sites start using a spec, and enough Web browsers start supporting it, it becomes “standard.” This is what Microsoft used to do with Internet Explorer — back when it had 90%+ share, it could simply say “this is how IE does x” and every Web site in the world would have no choice but to follow. Now, with Mozilla Firefox, Google Chrome, and Apple Safari all having sizable minority market share, everybody can jockey for their own definitions.
Keep this in mind when companies talk about their products being compliant with HTML5 — or any other Web spec. What they’re often saying is “we tested it in the latest version of our preferred Web browser and it seems to work right.”
Is it really going to work on the Web browser you’re using on your computer, tablet, or phone? You’ll have to try it yourself to find out.