Thanks for the quick diagnosis. I will report this error in their page to upstream, and no doubt they will fix it quickly.
So it seems Chrome (and presumably other popular browsers although I didn't check) are in violation of spec and ignoring the given base href. As a matter of robustness, perhaps uscan could, in the event that using the given base href fails, try again with the default base href? I suppose it depends how often this situation arises in practice.
