The definition of an 'applicable specification' is marked as only having relevance for implementations: http://www.whatwg.org/html#other-applicable-specifications
I think that the paragraph which defines an applicable specification and the note that follows it are relevant to authors, too -- in particular authors wondering if they can extend HTML. Currently 'HTML5 for Web Developers' has a link 'other applicable specifications' which doesn't go anywhere: http://developers.whatwg.org/elements.html Cheers Smylers
