I would really love to not have to download an extension or another app to read EPUB books. Just like PDFs can we have one to read EPUBs natively?
Further we can have custom fonts, margins and dark mode to make it a really good reader along with syncing bookmarks/notes for the books between devices/systems. In my case it's the same storage but between Windows/Linux.
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I like your suggestion and also looked at igorlogius' link. The problem I see is that such a reader would only work with files with no Digital Rights Management attached. While the universe of "open" Epub publications is by no means empty, virtually all the major publishers protect their publications in all reader formats. So, no browsing Barnes & Noble or Kindle, purchasing a book and immediately reading in your browser. I suspect the Mozilla Organization has little to no interest in breaking DRM protection or assisting anyone else in doing so. The homepage of Igorlogius' link states that it only reads non-DRM Epubs.
Baen books eschews DRM on any of its publications and also makes them available in other reader formats than Epub. Many to most self-published content is DRM free, as is anything in Gutenberg. There are several other organizations that are archiving materials on which the copyrights have expired; where the source material is suitable for conversion, "open" Epub is almost always available.
Browser supporting EPUB actually more make sense than supporting PDF. PDF is a standalone binary format. EPUB is a collection of files that already supported by a browser. Like (x)html, css, common image formats, etc., in common ZIP (extension renamed) archive which also can be decoded/decompressed by browser (it's built in).