My thoughts regarding HTML as an alternative to this:
1. AFAIK, there is no standard way to bundle a webpage containing images into a single file.
2. We have EPUB, and my experience using it has been horrible. Either the format is bad or somehow every single reader I've used sucks, and I've tried many (Foliate and the now defunct Readium Chrome extension are the ones that suck the less, but the experience is still much worse than reading a PDF)
At this point, solving the bundling problem of HTML (and using normal web browsers) seems like a better course of action than trying to use EPUB.
3. Text that always fills the width of the screen sucks if you're using a screen bigger than 10 inches.
Being able to coerce a document into a page whose border is clearly delimited (just like PDF readers do it) without having to resize windows is, in my opinion, an absolute necessity. Something like:
body {
border: solid black;
max-width: 25em;
}
Epub readers allow defining widths (or semi-equivalently: margins) but they always look bad because there isn't a visual boundary.