MathML in Send to Kindle EPUBs: what renders, and what breaks reflow

[dropkind]

I convert arXiv papers (from their LaTeXML HTML) to EPUB and send them to my Kindle, and tested how Send to Kindle handles MathML. I could not find this written down anywhere, so here it is. Disclosure: I build a converter for this, but the post is about the Kindle side, so no link.

Setup: Kindle Paperwhite (2024), US account, EPUBs sent through Send to Kindle, September 2026.

1. Personal documents arrive as KFX. Every document I sent, EPUBs with MathML included, is stored as documents/Downloads/Items01/<title>_<id>.kfx. So they take the Enhanced Typesetting path, the one Amazon documents MathML support for.

2. What renders. Inline and display math, fractions, roots, sub- and superscripts, under/over scripts, mmultiscripts, mtable with mlabeledtr, maligngroup/malignmark, mfenced, mpadded, mphantom, ms, mspace, mtext, mstyle, semantics. All render as text and reflow, and the font size changes as in any book. I tested with a small synthetic book (one section per group of elements) and with "Attention Is All You Need" (arXiv 1706.03762), which reflows like a store book.

3. menclose breaks the whole book. Amazon lists menclose as supported, but one menclose anywhere in a personal document makes the entire book open as a fixed page: font size locked, looks like a PDF. I found it by splitting the test book into one book per section, then one book per element in the failing section. Workaround: unwrap menclose. You lose the box or strike-through and keep the expression. If your maths EPUB arrives non-reflowable, search it for menclose first.

4. Not tested: annotation (I strip it, so formulas are not printed twice) and the five elements Amazon marks unsupported (maction, mglyph, mlongdiv, msgroup, mstack).

This is one device and one region. If you have a Scribe, an Oasis, the iOS or Android app, Kindle for Mac or Windows, or a non-US account, a report would help: send yourself an EPUB with MathML and say (a) whether the font size changes and (b) whether the formulas render as text. I can attach the synthetic test EPUB if that is useful. I will add the results to this first post.