Give each movement a job
Decide whether a transition communicates hierarchy, a state change or a brand moment. Remove motion that does not support one of those purposes. Keep the main action available even when a visitor scrolls quickly or skips an introduction.
Design the resting state
A page should settle. Continuous background effects can consume resources while adding little information. Inspect the site while idle, after an interaction and in a hidden tab, not only during the impressive opening sequence.
Keep a complete quiet version
Reduced-motion and fallback states need the same content and navigation as the full experience. Test them as designed versions of the product. A static image should not cover the only link or make the page depend on a renderer starting successfully.