Browsers will treat all versions of HTML as HTML 5
After joining the W3C HTML Working Group a few weeks ago I've been spending some time every day reading the discussion on the public-html mailing list. I have also been reading the WHATWG HTML 5 Working Draft to get a better understanding of what everybody is talking about on the list. It can be quite hard at times.
One thing that I simply could not understand at first was the proposed design principle once called "Don't Break The Web" and later renamed to "Support Existing Content", which is often referred to. I found it really confusing. How could anything break just because a new version of HTML is published? Browsers would still treat old content the way they do now, right?
Or so I thought, but that is not the plan. From the Conformance requirements section of the WHATWG HTML 5 specification (which has been proposed as a starting point for the new W3C HTML specification):
Web browsers that support HTML must process documents labelled as text/html as described in this specification, so that users can interact with them.
So, as soon as a Web browser claims to support HTML 5, it is required to treat all content served as
text/html (which means all HTML and nearly all XHTML) as HTML 5.
I'm not sure if I think that is actually a good thing or not, but it does explain what "Don't Break The Web" means.
Anyway, I hope that helps clear things up if anyone else was confused by this.