This can easily, and has, cause seizures. I've read two stories about children having seizures from this jump-scare website. It's not funny starting a website like that. 

The owner of Agor.io made the site name agor because many people play on Agar.io. If a kid who has epilepsy misspelled the site's name they would get a Jeff the Killer jump scare which may cause seizures.

I think if the site isn't removed, the owner should be sued. If the owner is a child, sue his parents. 

One of the seizure children's parents wrote on Reddit.

Ps. The video is not mine, it's just for representation.