I'm pondering the best way to let users add special characters (any character missing on their keyboard) in a TEXTAREA.

The simplest solution is to just list the special characters on the page and let the user highlight, copy and paste them. However this requires a few precision mouse operations, especially with small characters.

A more user-friendly idea might be to let the user click on the character and have it inserted automatically at the TEXTAREA's caret position (like the smileys work on this forum). The disadvantage with this is javascript dependency, as well as lack of browser support for the existing solutions.

Any other ideas?

Fundamentally, it shouldn't be the job of a webpage to provide basic user operations. But if for some special purpose you want to provide a set of characters the particular users might not normally have, I can't see there should be any problem using javascript to insert them. (I don't think there's actually any other way.) At the same time, you could always allow html character-entities or similar plain text codes.

Incidentally, what particular "set of characters" are you thinking of? Swedish for non-Swedish users, or something like that?

I was mostly thinking of symbols like ™ © ° etc. In cases when you don't know which ones the user needs maybe it's better to just link to some external resource, like http://www.htmlhelp.com/reference/html40/entities/

Maybe it's possible to facilitate the manual character highlighting (for copying) somehow? Making the font-size larger is one obvious idea.