Since EPUBs are just zipped HTML files, I guess rendering can be done more efficiently by the browser itself rather than by custom rendering on an HTML canvas.
most epubs are just zipped html, but not all.
There are different versions to this file format and some need to be parsed as xml. The chapter files will mostly adhere to html with caveats wrt anchor tags, image sources and similar as well as metadata wont be parsed/work with html parsers.
You could even - technically - take a picture as an input and then render it via background color rgb() using divs pixel-by-pixel. thatd still fall under that description.
It's still useful since browsers don't come with built-in EPUB support.