I haven’t used one before but there’s a couple of really simple options you should consider:
the spellcheck html attribute will allow the majority of users to see mispelled words and to fix them, it’s only the third requirement of validation that’s difficult.
JavaScript doesn’t expose an API that I know of to detect misspelled words, so if this is a must have the only option is to have large dictionaries of words either on the client or server(with ajax) and run the text through it.