Even if everyone is using new browsers, HTML5 is still not required to do anything that can't already be done using some other method.
They are promoting HTML5 over current industry standards, and so far it's been rocky. Example: Google actively promoting Flash support, but dropping video codecs. It's all promoting what is good for their business. Apple and Microsoft do the same.
Knowing the clowns who run Git, I would be hard pressed to rely on them on a service of this magnitude. However, if it was for a hobby web application, sure, I'd try it.
In reality, most developers will stick with HTML4+JS+CSS and use Flash if they want a canvas and timelines.
Most developers will wait for HTML6. No one needs an HTML5 'Movement'. That is ridiculous and shows ignorance in the work professionals do everyday in developing online apps and websites.