saw it on hacker news but it's already closed for comments.
Feedback for Gitbook Editor 5 beta
[specifically tried gitbook-editor-5.0.0-beta.6-linux-x64.deb on lubuntu 15.10]
Very nicely done WYSIWYM! I'm author of mathdown.net & https://github.com/cben/CodeMirror-MathJax/, which only renders math, and less well than your implementation.
(the exact meaning of "WYSIWYM" is fuzzy; styled text is probably not exactly what LyX meant but math/image/link rendering brings it closer, and in any case it's a better description than "WYSIWYG". I understand the term never got widely known and you might prefer WYSIWYG for publicity...)