| PASS Custom effects with no fill do not fire with progress = 1. | |
| PASS Forward-filling custom effects fire once with progress = 1. | |
| PASS Backward-filling custom effects fire only once with progress = 0 during the before phase. | |
| PASS Custom effects are called when the associated animatoun is paused and seeked. | |
| PASS Custom effects fire callbacks with no other reason to update the page rendering. | |