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Description
This is mostly a CodeMirror problem.
Viewing pages works on Android and iOS (don't know about Firefox OS and Windows Mobile but should work).
Clicking inside tends to zoom in a lot (at least on android), making it not very usable. There are stackoverflow answers claiming setting initial font big enough prevents this (perhaps it's even enough to enlarge font of the invisible textarea?).
Editing is even harder, because of CodeMirror might not reliably get presses from soft keyboard, because of the inconvenient zoom, and because of lack of various keys: arrows (should implement click to edit math cben/CodeMirror-MathJax#9), codemirror shortcuts and annoying to type markdown punctuation (toolbar #64 would help).