Beware:
Avast has decided today to FORCEFULLY push their Chromium-derivative browser to ALL users of Avast Free Antivirus (they seem to use a staggered deployment - it's not installed for all users at the same time to avoid overloading the servers). The Avast Forum is full of complaints about this sudden, almost malware-like appearance of the new program on users' desktops.
while Google Chrome supports U2F, Avast's fork of it does not support U2F but even so it announces itself with almost the same User-Agent as Chrome.
as a consequence, the incompatible browser warning from
https://demo.yubico.com/u2fis not displayed as the site thinks the browser is U2F-compatible. It's not.
U2F functionality seems to have been nuked out of Avast's fork of Chrome.Posted a complaint here
https://forum.avast.com/index.php?topic=184628.0