The DOM specs now use Document instead of HTMLDocument.
A tiny nitpick:
"For historical reasons, Window objects must also have a writable, configurable, non-enumerable property named HTMLDocument whose value is the Document interface object."
IE11 reversed their implementation to match WebKit/Firefox. I think we should update the spec instead.
Could you maybe email email@example.com with some more details? Or alternatively file a bug on the HTML specification.