I am looking for a WYSYWIG JavaScript editor for MathJax. I've read all the answers to similar questions (here and there) but could not find anything that is licensed under a liberal open source license (GPL/LGPL is great but not an option for my project), written in JavaScript, and working in WYSIWYG mode (instead of live rendering of LaTeX).
If such a thing does not exist, how would you go about building it?