HTML 4 and HTML 5 both define method as either "get" or "post", mapping to GET and POST, with no other valid options. This compliant behavior is widespread, which is why Rails uses Javascript magic to fake the presence of a method POST or GET parameter in some of the built-in magic. (I only know this because I've special-cased it for an app I built.)
Though, it should be noted that at one point HTML5 did plan to support PUT and DELETE, however that was then removed[1] again. Could be part of the confusion with some people.
I would reference when it was added, but since it's a working draft spec, it's been removed and I can't find the original addition date - but I believe it to be mid to late 2009.
My code (I've tried it with both upper-case and lower-case DELETE):