This doesn't appear to be alternating between the image and the alt tag. Rather, the image in question has some sort of cross-fade animation on it that is interacting poorly with the popup that is placed on top. Without any accessibility and safety fixes, the cross-fade animates smoothly (albeit probably should not be on top of the popup) — perhaps with the reporter's modifications the animation is broken, causing the strobe effect.
Bug 1658842 Comment 1 Edit History
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This doesn't appear to be alternating between the image and the alt tag. Rather, the image in question has a cross-fade animation on it that is interacting poorly with the popup that is placed on top. Specifically, they are animating the z-index between -100 and 9000, and the opacity between 0 and 1. The popup has a z-index of 1001. Hence the animation "through" the popup. This feels like a site issue we probably can't do much about. Without any accessibility and safety fixes, the cross-fade animates smoothly (albeit probably should not be on top of the popup) — perhaps with the reporter's modifications the animation is broken, causing the strobe effect.