when I insert xml:lang="en" lang="en" into the <html tag, I do not get any error from either Flight Crew or epubcheck ver 1.2.
Those attributes
should be allowed in the <html> tag, as you probably already know. How are you validating the epub?
As for the other error, might it be something in the CSS quote1 selector?
But if, as I suspect, the first error is due to a bug in the validator, maybe you should try to resolve that first. The other error may go away also.