Delete CSS and replace it with Mozillas UI system, XUL.
Get rid of its XML-like faggotry because pure XML fucking blows ass.
Update XUL to support similar CSS rules system.
Kill anyone that mentions XHTML.
Create separate animation system, completely separate from CSS/XUL.
In fact, go as far as making HTML5 animation an actual separate file format as well.
One of HTML5s main reasons for existing was to replace plugins and Flash in particular.
It DOES NOT AND WILL NOT replace Flash in any way that it exists right now.
Any HTML5 project will absolutely be 150% larger than a similar Flash project.
HTML5 doesn't support binary in its files. It can WORK with binary, but you dare save binary in an HTML (or JS) file and it will shit itself. The closest I got was base91 encoded data that doesn't break JS parsers.
Making a separate animation file format with access to a limited CSS subset, JS subset, SVG, Canvas, and embedded binary data, with native support for webms, webas and webp, and even some open archive format.
Web workers have their own JS subset, if you are wondering.
Delete the source code and spec for the DOM system and rewrite it from scratch for a new completely incompatible JS version.
JS Harmony isn't compatible with old JS parsers, so there is no reason not to.
The DOM is provably the worst part of webdev because it is a slow piece of bloated SHITE.
On the JS part, get rid of redundant fucking commands and features.
x[y] or x.y, fucking pick one.
Force full paths for commands, none of this short-path bullshit unless you are using something like LET. (which itself needs to be fixed because...)
Enclosures also need to be fixed. The overhead they cause is INSANE. When you go 3 layers deep, it becomes unbearable if you have any reasonably complex project.
tl;dr
Delete all specs except HTML and start over. Make an actual HTML5 animation file format to replace what Flash was used for, don't fucking add animation to webm!