I guarantee you that there are thousands of people working at Google who are:
1. Smarter than you
2. Know more about web standards than you
Seriously, get the hell out of here.
Deleted comment
And even if there are valid reasons for invalid markup, there is no reason to have DOCTYPE of the actuall standard if you are not going to follow it.
Sure there is. They want browsers to render the page with the latest standards support, even if they have a few trivial violations of that standard.
Browsers don't look at doctype and then choose what to support and what not: they will always try their best. See for yourself: http://kod.as/lab/html5/html32.html —that's HTML5 code with <canvas> but with HTML3.2 doctype. Try it in modern browser, canvas still works.
Let me repeat: browsers will always try to render all they can on the page, no matter if that feature was or not in the version of HTML page claims to be. The only thing doctype affects is rendering mode. Any unknown doctype will trigger browsers to render the page in standards compliant mode. This feature is the reason why HTML5 has a doctype declaration at all. If you use XHTML5 (i.e. use xml markup and server your document with MIME application/xml+xhtml) you can omit doctype, because MIME type then tells browser which rendering mode to use.