This is a regression caused by the fix for bug 36401.

Steps to reproduce:

emacs -Q
M-x browse-url-emacs RET https://www.gnu.org/software/emacs/ RET

Expected results:

The HTML contents of the Emacs website are shown, and the buffer is in
mhtml-mode.

Actual results:

The HTML contents of the Emacs website are shown, but the buffer is in
fundamental-mode.

The proposed solution to accomodate both wanted behaviors is to extend
the regular expression to cover a "DOCTYPE" in a case-insensitive
way. The standard says that's also supported:
https://html.spec.whatwg.org/multipage/syntax.html#the-doctype

I've attached a patch with the proposed solution, on top of
emacs-27. Let me know if you'd want to apply it on top of master
instead.

Thanks.