Flashing lights can cause seizures yes. That's why you'll often see a warning at the start of more visually frenetic videogames.

Famously an old Pokemon episode gave several hundred kids seizures. The wikipedia article contains the scene if you're feeling particularly brave :) https://en.wikipedia.org/wiki/Denn%C5%8D_Senshi_Porygon recommend not watching fullscreen and being ready to click away, as it is a bit intense -- I'm not epilectic but it makes me feel _weird_.

I also remember back in the day learning OpenGL I accidentally wrote a program which would flash between my desktop and a solid color at 60hz. It was literally painful to look at in the few seconds it took to shut it down.