If I am not missing something, this doesn't match common practice.
Could you elaborate a little bit?
Isn't mozXXX or webkitXXX more common? Or is the spec referring to constructor methods here (in that case, that doens't seem to be relevant in this spec unless it's a subclass of XMLHttpRequest or the other) ?
Fair enough: http://dvcs.w3.org/hg/xhr/rev/ac2bf114fd20