1. effects from your eyes (from looking at the strobing image)

2. PWM brightness controls causing your recordings to look like they strobe

The site in the OP wasn't working, so I tried a demo site instead: https://www.da.vidbuchanan.co.uk/strobegate.html. I looked at the strobing image, closed the tab, and sure enough my monitor (IPS, 60hz) was showing some signs of flickering. It disappeared after a few minutes. However, I performed a second test where I opened the site and didn't look at it with my eyes. The strobe after effect did not show up. Even though I didn't look at the monitor, I verified afterwards that a strobe was shown by a phone video recording. I also did a slo-mo video recording of my monitor without any visible strobe effect, and there was visible strobing in the recording, probably from the shitty PWM. You should follow a similar procedure to rule out whether it's something with your eyes or not.