"The problem we're facing right now is there is already a lot of excitement for HTML5, but it's a little too early to deploy it because we're running into interoperability issues," Philippe Le Hegaret, W3C interaction domain leader, told InfoWorld. "I don't think it's ready for production yet. The real problem is can we make [HTML5] work across browsers and at the moment, that is not the case."
Of course, the term HTML5 encompasses a lot, and numerous parts of it are at various stages of development, and thus no single Web browser supports it completely. Browser vendors have their own timeline to implement different parts of the specification, and only the latest alpha and beta versions have made the most progress.
HTML5's development began in 2004. W3C's goal is to have it feature-complete by mid-2011 and have final approval in the next two to three years. Even then, the specification is an ongoing effort. Interoperability is always going to be an issue for Web developers, though it may improve once HTML5 is adopted everywhere. Nevertheless, there are still going to be many issues in the coming years before we reach that day.