I was actually just thinking about this when I saw this forum loads in quirks mode. It's something I've been contemplating for a long time after I started using html5 since html5 really did drop doctypes altogether, and the specification only recommends you have one for legacy purposes, i.e. standards mode.
As far as I know Firefox, Opera, and Chrome, and I would imagine other browsers aside from IE, don't actually use the incorrect box model if the page is loaded in quirks mode.
The x-ua-compatible header that most people usually include anyway for chrome frame triggers standards mode in IE8, but not in IE7.
So I guess it comes down to two questions.
Is dropping the doctype conforming? and are people willing to stop supporting IE7? (Most websites probably don't look so good in IE5 mode. Personally, my website barely even works in IE7.)
As far as conformance to the spec goes: <dev.w3.org/…>
"A doctype is a required preamble." It then goes on to further explain that it is only required for legacy reasons. Of course, a page won't be recognized as html5 by the validator without the legacy doctype, but that's all doctypes were useful for anyway, which is why I imagine were dropped. So, ironically enough, html5 isn't recognized as html5 without a doctype.
Maybe this is more a discussion for the public-html mailing list, but it was on my mind and I thought I'd start a discussion here.
What do you guys think?