Hi Simon,
Thanks for your report.
After extensive research, I found that the W3 Validator does not yet support the @supports not selector(:focus-visible). Apart from that, it has nothing to do with the variables.
However, it is generally a “widely available” feature and is given as an example in the MDN docs, so it shouldn’t be a problem, and, in my opinion, the parse error can be ignored.
Best regards,
timmse