I definitely prefer to read research papers in html. I like to zoom in a lot when reading a long piece on my computer since it helps me read faster and keeps me from getting distracted. I've been thinking about working on a side project where I convert pdfs to html for academic papers.

One benefit of PDFs for research papers is that you can easily save them to your own computer, build up a library of them, highlight lines with functionality built into most PDF readers. I generally prefer HTML for reading, but PDF has some benefits, too. Granted, most of these features are also available for HTML. But for some reason you need to look for browser plugins in order to highlight HTML pages, whereas in PDF you can just use the feature. And PDF is always about the content whereas HTML also typically contains navigation and other distractors.

Regarding a library of HTML documents: https://github.com/gildas-lormeau/SingleFile#install

I have installed an extension named "single file" in my browser, which allows me to save any webpage, as it looks right now in my browser, as a single HTML file. Images and CSS is inline, javascript (I think) is removed. Quite handy when you prefer a folder and file based workflow.

What about epub? Under the hood it is basically html, but viewers know to treat it as a written work.

Epub is ok, but it has no support for math equations (practically all implementations just dump them into raster images) and HTML's typography leaves much to be desired.

There are plenty of good reasons why TeX and LaTeX are still the workhorse of scientific publishing in spite of the emphasis on fixed format layouts.

It's much more restricted than html, even less support for animations than pdf.

I really don't get why mhtml has been discontinued by browsers??

It hasn't. Chrome can still save .mhtml just fine, and will also open them if you copy/paste the path into the URL bar.

What about Firefox ?

It never had support for mhtml, only through an extension that's no longer supported.

Hmm, I might be confusing with Opera ?

Somehow related: I find Snappy Snippet extension (for Chrome) very interesting. It's supposed to let you make a "live" screenshot of a DOM element. Unfortunately, I've not tried it much as I only rely on Firefox in day to day browsing.

https://github.com/kdzwinel/SnappySnippet

I currently have Adobe's Creative Cloud All Apps, which includes Adobe Acrobat. I've bought tons of books on Java, Javascript, HTML, C++, etc.

Ugh, I really hate when I stumble upon a research paper only available in html and not in a standard two-column pdf - I find it much harder to read in general