In a simple sentence: HTML is going to "unversioned model", according to this there is definitely a concern: OK, living standard? Does this mean the standard could be changed/updated/revised at anytime? I saw one person asked posted this question and Ian Hickson replied, he emphasized WHATWG worked hard on backward-capability, and they worked tightly with browser vendors to make sure they do not change things that most people depend on, this is good and really important!
So, things is getting more and more interesting with HTML 5 (should I just call HTML?), few days before (18 January 2011) W3 just "unveiled" the HTML 5 logo: http://www.w3.org/News/2011#entry-8992, I bet there could be more or less confusion:)