Amazon provides software, called Kindle Create, that can be used by self-publishers to convert Word documents for Kindle. Any books produced using that tool will be in KFX format and will support Enhanced Typesetting. However Kindle Create is limited in the formatting options it supports.
Other than that, publishers submit their source files to Amazon and Amazon automatically runs kfxgen on them to produce a copy in KFX format. The conversion process can fail if there are problems with the content. It is up to the publisher to correct and resubmit the book, if that happens.
Kfxgen is also built into the Kindle Previewer. Publishers can use that to test if their books will support Enhanced Typesetting without having to submit them to Amazon.
The older kindlegen software, which produces MOBI and KF8 formats, is more forgiving. It will accept content that might display incorrectly or inconsistently on Kindle apps and devices. So just about any book will be made available in those formats, even if it has problems.
Some examples of content that will pass kindlegen but fail kfxgen are: complex SVG images, complex or nested tables, tables with negative margins, content in unsupported languages, unsupported CSS (eg: counter-increment, first-letter selector with non alpha text), unresolved internal links, and non-numeric page numbers.
The
Amazon Kindle Publishing Guidelines does a fairly good job of stating what is and isn't supported.