Thank you.
I've tested: 3.0 works for sure, as should specifying no profile at all, because the css valiator uses the 3.0 profile by default.
Might I suggest: pass the file just as you now do to the validator, but open the browser session on the "upload file page" --
http://validator.w3.org/#validate_by_upload -- so that the user can choose the option there. This might add as little as one additional click to the validation process (if using the 3.0 default), but you would never have to deal with this issue again; Sigil's code could remain static in this regard. You guys work hard enough, so why not let someone else do the maintenance, when new standards are adopted?