> correct behavior on windows size change
Except the PDF is not responsive at all and you can't increase or decrease the font size without increasing the whole width of page.
> Some vendors have switched to online-only for some documents and it always annoys me.
HTML shouldn't mean online-only. If the vendor isn't trying to make it hard to download, you should always be able to convert to PDF. But PDF to HTML is very hard or impossible.
In any technical/scientific document I do not want to increase or decrease the size of any element, e.g. of one of the fonts.
You only want to do an overall proportional zoom, when needed.
A well-designed document page has appropriate size ratios between various kinds of texts, formulae, tables and images, which should not be corrupted by changing the size of a single element.
The pages where the author has not formatted them adequately are ugly and hard to understand, which is what you typically see when this kind of content is written as HTML/EPUB documents, which are rendered non-deterministically.
Lazy writers may like HTML, but readers who must read and search through vast amounts of technical documentation do not like it.
There are many good PDF readers that are adequate for reading and searching even huge documents, but I have never seen any tool that works acceptably for EPUB/HTML big documents, which is not surprising because no tool can compensate the fact that the writer of the document did not design the layout of the pages carefully.
I absolutely want to increase or decrease the font while keeping the whole width unchanged. This is not possible with PDF. Or maybe it is, but I've never seen a PDF file that supports it. I may be reading on a tiny phone or on a FULL HD monitor. Or I may want to put the file on the left side of the monitor and something else on the right. If I'm on a huge monitor, I might not be OK with tiny fonts. I actually like tiny fonts sometimes, but other times I want to zoom everything. I think at night, where my eyes are more tired and when I have shut off the lighting in the room, I prefer bigger fonts.
I don't particularly care about increasing fonts ONLY. I've mainly done that on Firefox for Android years ago. The standard browser zoom (with CTRL++, CTRL+- or CTRL+WHEEL_UP, CTRL+WHEEL_DOWN) increases everything proportionally. The exceptions are newer websites that try to cram too much logic about what should be zoomed or hidden at a given level of zoom, but I'm not talking about 5 MB SPAs, but normal HTML sites.
I have read vast amounts of docs and I prefer HTML. So OK, I agree that some people want something unresponsive that looks like a printed page. But I think the issue is with how unpolished most EPUB readers are, not in the format itself.
For "underetministic" rendering, it's usually a PDF that had 2 columns, placed an image on the bottom of column 2 and referred to it from the top of column 1. If you automatically make an EPUB from that, the image would be far from where it's referred to. But nothing's stopping you from putting an image right after the paragraph that refers to it.