On the EPUB 3 W3C Community Group, there was recently a new discussion about the support of MathML by EPUB readers. Such discussions often mix the situation in e-readers and the one in reading apps using browser engines.
Most e-readers develop their own rendering engine and choose their battles carefully; MathML support is so complex that many don’t spend time on it. On the contrary, reading apps based on browser engines (like Readium toolkits) support what the underlying rendering engine does, and most can can alternatively integrate the open source MathJax library.
MathJax is supported by Readium JS (web reader) and will soon be supported as well in the new Thorium Reader (Windows, MacOS, Linux).
MathML support by browsers was discussed (again) last week at the W3C TPAC in Japan: This post is a quick summary of what I learnt there: