User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:90.0) Gecko/20100101 Firefox/90.0
Steps to reproduce:
I've shortened the image names but apart from that I used this HTML:
<link rel="preload" imagesrcset="small.jpg 100w, medium.jpg 500w, large.jpg 1000w" imagesizes="100vw" as="image">
The image did not preload.
I think this line "One or both of the href or imagesrcset attributes must be present." from https://html.spec.whatwg.org/multipage/semantics.html#the-link-element means the href can be absent and the preloading will still work. I tested in Chrome and Edge and it works there.
In Firefox I have to use an empty href i.e. href="", which the spec disallows if I'm reading it right.