Enhanced Typesetting is a series of typographical and layout features that improve the display and readability of content across Kindle reading platforms.
Enhanced Typesetting is applied only to reflowable format Kindle books (uploaded in EPUB, MOBI, DOC, DOCX, or HTML). We're continuously working to make Enhanced Typesetting compatible with more titles and will automatically enable enhancements for your book when possible. If it's enabled for your book, you'll see "Enhanced Typesetting: Enabled" on the book's detail page.
For more information on Enhanced Typesetting, please see Enhanced Typesetting for Better Reading
and Appendix A of the Kindle Publishing Guidelines