The slides note that the new HTML5 doctype works in current browsers. Does it trigger quirks mode in older browsers (e.g. pre-2010 releases)? If so, that essentially makes it a lot less usable that the slides would suggest.
The HTML5 spec is written with backwards compatibility in mind, and the DOCTYPE is one of the examples. HTML5 specs current behavior and implementations. The DOCTYPE would’ve been different if browsers didn’t already support it.
I realize a lot of additional info is lost on people who are viewing the slides but couldn't attend the presentation, and am working on a blog post with some more explanations.