Version 1.0 patch-notes:
Added furigana markup functionality
Writing <漢字>[かんじ] results in 漢字, while writing <漢字>{かんじ} results in 漢字.
The markup is the same as in the Easy Furigana userscript by @Kumirei.
I hope you never want to post the character sequence < something >[ something ]. If you come across such a situation, you have to disable IME2Furigana (with the F button).
IME input gets converted to furigana markup
Instead of immediately generating ruby markup and cluttering the input box with a monstrosity like <ruby lang = 'ja-JP'>振<rp>(</rp><rt>ふ</rt><rp>)</rp></ruby>り<ruby lang = 'ja-JP'>仮名<rp>(</rp><rt>がな</rt><rp>)</rp></ruby>, the IME input gets converted to furigana markup: <振>[ふ]り<仮名>[がな].
Button state is now persistent
The button state should now get shared across tabs and even sessions (unless you clear the browser data).
Important change: If you disable the IME2Furigana functionality with the F button, it does not only disable the IME input to furigana markup conversion, but also the furigana markup to ruby conversion.
If I broke something please also let me know.
Link to previous script version for downgrading in case version 1.0 doesn’t work/isn’t to your liking