Using motion behind static text creates a depth mismatch that can be triggering. Even subtle animations can be harmful when placed directly behind the text. What constitutes excessive motion can be subjective, but a good rule is to avoid motion that takes up more than 10% of the central visual field.
Excessive motion can trigger vestibular disorders and seizures. To reduce harm, it’s important to include a motion warning if your site uses excessive motion, or if you’re linking to a page that uses excessive motion.