Radiance
Lamps in a dark room, traced to whatever they light
A dark room with a few small lamps in it, and everything they light. Shadows are hard beside a small lamp and soft beside a near one, and turning bars sweep shadow blades across the floor. The pointer is the brightest lamp in the room, so moving it moves every shadow.
Rendering cost: heavy. A raymarch or escape-time loop per pixel: cap the pixel ratio, or keep it to a card rather than a fullscreen background. How to tune it
Take Radiance with you.
Radiance is free. Sign in and it is yours — the background itself, with your settings already saved in, ready to drop into your site. The account is only somewhere to put the files. No card now, and no card later.
The other 61 come with a licence, $9.99 once.
$0forever
Your preset is in the URL. It survives the trip.