Hi there,
sorry for the late reply…
Eye tracking usually works on people with glasses.
However, in regards to people wearing masks or other things on front of their face, it should work as long as both eyes are visible for the Eye tracker.
Tobii has not specifically done tests with caps or masks. However, looking at our Assistive Technology, we do supply Eye tracking for individuals with communication disabilities, which is some cases also wear masks or caps.
However, just try it out and if you want, and please share it in the forum.
Regards,
Konstantin