Earlier quoted context omitted.
HTML isn't immune from broken backwards compatibility. The frame element has been completely removed, the hgroup element is gone, and so is the dir element. acronym is deprecated in favour of abbr. isindex, plaintext, xmp, and listing are all dead. The attributes border, clear, background and bgcolor have all been removed by HTML, and shifted to be CSS' responsibility instead. Just moving between EPUB 2 and EPUB 3, y…
Yes, but all of those deprecated elements can still be rendered just fine by some of the current browsers. And even if, say, you couldn't display an index page with frameset, then the stuff inside the frames is just plain old html files anyway, which you can display easily. bgcolor, etc. all still render fine in current browsers as well. The main thing is that information which is important to people will get convert…
Vs. decent-quality old "dead tree" books, properly stored, can be ignored for centuries and still be perfectly fine.
You might want to ask a good historian or librarian about all the incredibly important (historically, to us, now) documents which we know existed, but we do not have, because there was no continuously-operated, high-budget "Holy Brothers of Document Preservation" monastery doing all the re-copy work needed to preserve them. (And maintain off-site backups in case of fire at the Monastery, and ...)
(If you aren't familiar - most of the materials used for documents in ancient times degrade fairly quickly. Unless (say) carefully stashed in a nice, dry cave in an arid climate. And even that stuff tends to be "crumble if you touch it" fragile.)