I accidentally opened the EPUB3 file in 7Zip, then realized you can just decompress the archive and open the .xhtml files in any standard web browser.
Seems like it would be fairly simple to have web browsers support EPUB3.
Seems like it would be fairly simple to have web browsers support EPUB3.
https://www.windowscentral.com/microsoft-edge-will-no-longer... (link found via Wikipedia article on "Microsoft Edge Legacy")