Your system asks for reduced motion, so VFX-JS draws nothing. Demos on this site show plain HTML.

import type { AddTransitionOpts } from "@vfx-js/core";

type AddTransitionOpts<T extends Transition | TransitionClass = Transition | TransitionClass> = Partial<TransitionSpec<T>> & {
    autoReverse?: boolean;
    keepWrapped?: boolean;
    slideSpec?: (slide: HTMLElement) => RunOpts | undefined;
    waitFor?: TransitionWaitFor;
}