Actually, it goes further than that. Everyone but IE seems to just return the value of the content attribute when you do a get on the IDL attribute:
<!doctype html> <script> var el = document.createElement("form"); el.setAttribute("method", "invalid value"); alert(el.method); </script> IE alerts "get", everyone else alerts "invalid value". Are non-IE implementers interested in changing to match IE here? IE's behavior seems more useful.