[Responder] David Baron
Making varying DOCTYPE syntax conforming will make it more likely that browsers to use variations in DOCTYPE decarations to trigger new mode changes, even if the specification declares this to be nonconforming. I believe authors would be more willing to use apparently-non-proprietary mode changing syntax than clearly proprietary markup or clearly-proprietary comment syntax. Having more modes makes it harder for new entrants in the browser market, as I described in http://dbaron.org/log/2007-03#e20070325a , http://lists.w3.org/Archives/Public/public-html/2007Apr/0279.html , and http://lists.w3.org/Archives/Public/www-tag/2009Aug/0054.html . Allowing variations in syntax that are conforming, but do nothing, seems likely to encourage authors to believe that those variations will do something. (Witness, for example, the large numbers of authors who change the "EN" within the HTML 4.0 FPI to their own language code.) Allowing DOCTYPE declarations that can be connected to DTD-based validators encourages people to use inferior DTD-based validation tools rather than a conformant HTML5 conformance checker.