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.