Is there some input method to easily type math symbols UTF8 characters (like U+2200 FOR ALL ∀, U+2203 THERE EXISTS ∃, U+2208 ELEMENT OF ∈, U+2190 LEFTWARDS ARROW ←, etc...)?
I would like to type easily these characters in some <textarea>
in Firefox (38 or 42) on Debian/Linux/x86_64 under xfce4 or Gnome (and I want to avoid Javascript tricks for that). I am currently copy/pasting them from some CharacterMap application, which is uneasy.
The answers to this related question requires some ad-hoc configuration; I am asking for a standard input method.