The 10-15 year time frame includes the time required stabilize the spec, produce the test suite containing thousands of tests and achieve interoperable implementations. Browsers are expected to implement HTML5 during this time, not wait until after. Indeed, they have already begun, which is why we’re already seeing early implementations of video, audio, and we have had canvas for a long time already.
For comparison, look at CSS 2.1. It’s been 10 years since CSS2 was started and that’s still only a candidate recommendation. However, it’s been relatively stable for a few years now, while still being updated based on implementation feedback.