The 'official' file extension for Kindle is
.AZW. Closely related to
.MOBI and
.PRC.
Since Kindle has up to 80% of the market, seems a mistake to not offer one of its flavors.
(Edit) I just noticed in that blurb for .MOBI that it supports Javascript. Does that mean that I could use Javascript to manipulate the CSS in a Kindle? That would be cool because it opens up the possibility of adding easter eggs to the text. Blocks of new text/graphics that can be opened and closed.