ELI5, how can you tell if flashing in videos is severe enough to need an epilepsy warning?

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Prompted to ask because some channels I watch on YouTube put warnings, and some don’t… But the “severity” of the flashing always seems very different in the channels that do put warnings.
I don’t currently have anyone in my life with epilepsy, but I definitely wish I understood what could be a problem for anyone I may meet in the future.

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