The enormity of the consequences of HTML only allowing GET and POST cannot be overstated IMO. It's maybe the most damaging technical decision in the web standards space - ever. I see HTML forms as a root cause for the WS-* "everything goes over POST" debacle, a billion dollar industry mistake, at best.
I've always wondered what's the history behing the GET/POST only restriction in HTML. Was there a good reason (or did something appear to be good reason) for doing so? I can't think of one.
HTTP 1.0 defined only GET, POST, and HEAD. By the time HTTP 1.1 standardized other methods, HTML was probably too bogged down in the browser wars to adapt.