Now apparently, your complaint is not that PG HTML does not make good .epub files, or that including a generic stylesheet "breaks" the ".epub", but that you don't like the .epub generator that Mr. Perathoner wrote. That complaint, with which I sympathize, needs to be directed to him individually; it cannot, however, be generalized to /all/ .epub files, only those created by his software.
First, it should be obvious to all the PG ePub is NOT simply HTML repackaged and compressed in that PG ePub is offered in two flavors, with and without "illustrations" and if those "illustrations" are illuminated caps then that is going to have at least SOME impact on the ePub files generated and the enjoyment or lack thereof of the end reader! My *complaint* rather was that YOU said it was not necessary to have access to Marcello's converter because I could easily create my own ePub files to see what my HTML would like as an ePub. Which was clearly false. My *suggestion* after *others* at PG complained that DP keeps turning out HTML which breaks when turned into PG ePub files was that maybe PG ought to offer Marcello's converter software in a portable form (I tried porting it but can't get it to work) so that DP authors (PP's) can actually TRY the ePub format as part of their content development process, and perhaps IF they saw for themselves that they were making choices in their HTML cutesiness that is causing the ebook readers experience to fail THEN perhaps they would make better choices. BUT, currently the only way to see how the ePubs or MOBI is going to turn out is to submit the completed HTML to PG for posting at which point in time its way too late to make more reasoned HTML design tradeoffs.