"how [does one] properly prepare HTML files for PG?" we can say, "here you go, follow these rules and you will be compliant, and if something doesn't make sense or isn't covered, we will clarify or modify the rules so it /is/ covered."
Your ideas for using xhtml "class" to mark semantics (which I do not disagree with the concept) then has the problem that many [x]html tools which volunteers may want to use to tackle the job don't support your "class" semantics. Thus volunteers are stuck using your tools, or generic text editors such as notepad++. What, in practice do the WW'ers do when presented with what looks like an otherwise acceptable xhtml which doesn't have your "class" semantics? What should DP do to implement this? Certainly having a well-defined marking scheme for "obvious" things like TOC, title, author, chapter headings, etc, would be an obvious huge step forward.
Development of a standard is primarily a political endeavor, not a technical one.
Thank you -- yes. The politics, frankly, lie between PG and DP. If they agreed to anything, they have enough "critical mass" [for better or for worse] to stick it to the rest of us, because they would have us outvoted 10 to 1. Again, if you want to win the politics, go get DP on your side. Without them, you are not going to get anywhere.