Kindles can read .mobi files, which is a pretty open format. Calibre can easily convert EPUBs to .mobi (or to the similar .azw3 format) for reading on Kindle. While Amazon has introduced recently a new format with advanced typographic features that is highly proprietary (.kfx), uptake of it among publishers has been slow.

Mobipocket has been dead for the last couple of years and I can’t find a spec anyway. The Library of Congress describe it as a “proprietary, partially documented, binary format for ebooks”[1] which doesn’t really meet “open” by my definition. As for KFX, that’s 100% proprietary, yes.

Amazon in the US at least has sewn up the ebook market such that they have no interest at least in implementing open standards. The best thing to do in these situations is to vote with your wallet.

[1] https://www.loc.gov/preservation/digital/formats/fdd/fdd0004...