Can you lazy load CSS background images?

It isn't available out of the box, but it can be done using IntersectionObserver.

CSS syntax for a single image:

--background-image-lazy: prague-3010406_1920_thumbnail.jpeg;
background-image: var(--background-image-lazy);

CSS syntax for multiple images using image-set:

--background-image-set-lazy: image-lazy-set(
      url("ekg_1280.png") type("image/png"),
      "ekg_1280.webp" type("image/webp")
    );
background-image: var(--background-image-set-lazy);