maybe html can do all of these and it will only cost me the time it takes to build the app, but right now PDF does all of those things for me, here today in my pocket, for $15. Which is nice.

I'd love to see a text to speech engine that pronounces formatting but I think it might be more complicated than learning to pronounce something boldly. Am I yelling? Am I keeping my voice low but adding intensity? Can you automate answering that question in a way that's mostly correct most of the time? If something is in italics am I whispering, stage whispering, emphasizing or merely saying the title of an existing work out loud? It's a fundamental abuse of a text formatting engine to try to use it for speech formatting, you either have to use the existing tags for things they were never intended for or you have to start adding tags like <slywhisper> and <scream emotion="angry"> vs <scream emotion="excited">. That being said, an html-independent form of emotional text annotation might actually be a good idea as the inevitability of synthesized human voices being a part of our daily lives takes hold.

I find PDFs easier to organize than HTML because HTML is any number of files referencing each other across a directory structure that can have any size or shape, and a PDF is a single file. If I'm searching my library for Bob Wilson, I want his books to show up and I want them to have his picture in them if that's how the book was published but I don't want Bob_Wilson.jpeg to show up as a result. I could automate print to PDF from html or use the tool someone else posted in order to condense my saved HTMLs to single files but that's more processing time and effort in order to get what I already have from a PDF

Syncing position across HTML files may be doable, but syncing position across PDFs is done. You're absolutely right that that has nothing to do with the format but the (implied) question I was answering when I brought it up was why I would sometimes want one and other times want the other. That's why.

Finally, and probably the only one that really matters inasmuch as all the other reasons can be coded around but this one cant: the places I get documents distribute them in PDF, mobi and epub but almost never in HTML