If you click a <button>, the "focus" event is not dispatched on Windows.
Actually this doesn't happen on Mac either, but that's because Mac follows the platform conventions (see https://bugs.webkit.org/show_bug.cgi?id=220243 for more details).
I have the feeling that Windows could dispatch the event like GTK+ and recently WPE (see r271671) are doing.
A patch to enable this on Windows would be very simple, similar to the one for WPE port. Some :focus-visible tests would stop to timeout once that's changed.
Anyway this is just for your information, in case you want to change that or not.