In part 1 of 2, Temani Afif shows how to speed up, slow down, and reverse infinite CSS animations on demand. He uses animation-composition: add to layer two animations, CSS variables for duration and a speed factor, and abs() and sign() functions to control velocity and direction independently, without restarting the animation.
frontendmasters.com/blog/how-to-control-infinite-css-animations-part-1-of-2