Web pages do not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds.
Do not include content that flashes more than three times in one second.
Prevents seizures in users with photosensitive epilepsy.
While with websites it's not a common practice to include flashing content, this effect might occur due to a problem with JavaScript or CSS. In those cases, multiple flashes might happen during a page load.
The flash size should not be too large. The flashing area should occupy less than 25% of any 10-degree visual field on the screen.
Red flashing is more likely to cause seizures. Especially when combined with high contrast.
Windows has a test tool: Photosensitive Epilepsy Analysis Tool (PEAT). I haven't used it myself, so I don't know how it works.
Check animations and hover effects to ensure that flashing isn't a byproduct when an effect doesn't render as intended.