A quibble with the footnote - the author has muddled the history about the WHATWG and W3C pretty badly. The W3C rejected the WHATWG's work initially in 2004; the WHATWG then created the HTML5 specification (largely by building on Web Apps 1.0 + Web Forms 2.0) as a browser-backed guerilla group outside the W3C. The W3C then adopted their work in 2006 (http://dig.csail.mit.edu/breadcrumbs/node/166), and a fragile partnership with the WHATWG has been in place since. The W3C therefore deserves little credit for the HTML5 spec - they declared HTML dead in 1998 after all, tried to turn the web into XML (via XHTML), and then eventually came back to earth in 2006.
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Surely it's also not simply being pedantic to insist that things like CSS3 (!) and SVG don't get lumped into the "HTML5" catch-all. Pity the poor designer or developer just getting to grips with this stuff who has to figure out what someone means when they say "HTML5".
I am aware of the history of WHATWG and you are right that its beginning was largely a reaction to the intransigence of W3C at the time. However, the current relationship is a healthy and collaborative one (I would certainly wouldn't describe it as fragile any more). I would have been remiss not to at least mention the WHATWG, but the footnote is already too long-winded and giving a complete history of the Web was not my purpose (others have done that better). There are many historical events that I painfully chose to omit because while they are personally important to me, they didn't actually add to the purpose of the post.
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I personally wish that there was a different term than "HTML5" to refer to the additional work happening around and on the W3C HTML5 foundation. But, unfortunately there is no replacement term that quite captures the full scope of what most people mean when they use the term "HTML5". I share your distaste: it has a version number in it for crying out loud, it can't refer to something vague! I considered trying to use my post to promote "HTML5+" or something similar so I would not contribute to the corruption of "HTML5". But I decided that was the impossible fight the popular usage and chose to instead pick a new term for the narrow reference "W3C HTML5".
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Surely it's also not simply being pedantic to insist that things like CSS3 (!) and SVG don't get lumped into the "HTML5" catch-all. Pity the poor designer or developer just getting to grips with this stuff who has to figure out what someone means when they say "HTML5".
Nevertheless, as Mark Pilgrim said: "HTML5 will continue to be popular, because anything popular will get labeled "HTML5"." (http://diveintomark.org/archives/2011/01/09/dive-into-2010)