Amazon's .kfx ebook format and predecessors are extensions of html5, css and backwards compatible with mobipocket (the 2000 French proprietary format that Kindle bought and used).

Proprietary format.

https://goodereader.com/blog/kindle/amazon-issues-new-mandatory-kindle-for-pc-or-mac-update-to-download-new-e-books

Read 2023-01-29T09:22:00-08:00

https://goodereader.com/blog/kindle/amazon-is-no-longer-allowing-downloading-kindle-unlimited-titles-via-usb

Amazon is no longer allowing downloading Kindle Unlimited titles via USB<br /> January 14, 2023