As a hobbyist, I certainly cannot provide a solid foundation for my views, other than that some things feel right, and some don’t. In this case, a lot of X/HTML5’s “backward compatible” features are plain 1996 déjà vu. Unquoted attributes. Mixed cases for tags. The I, B, and FONT tags — all of that in the name of what? A brain-damaged Microsoft HTML parser that is till unable to handle application/html+xml Content-Type??
XHTML2 might be have a few mis-designed features (an href for any element?), but X/HTML5’s built-in class names are simply moronic!