There is no built-in support for spell-checking.
The API would allow to integrate spell-checking of your own:
* Use the API to be notified when the user edits text
* Use the API to extract text from the DOM
* [Spell-check the text using the spell-checker of your choice]
* Use the API to highlight mis-spelled words, probably by inserting
<span> elements around the mis-spelled words to highlight them (and/or
by using the IWindowSelection interface to select the word[s]).