Would this be susceptible to tracking issues? E.g. place a hidden picture that loads WebP or PNG otherwise, to track which users support it and which don't
I would assume that’s entirely possible, but something like this could be done using the HTML5 <video> or <audio> elements for video or audio codecs (there’s even a DOM API to do it! [0]), and some browsers advertised WebP support via the HTTP Accept header [1]. For tracking, I’d assume it’s not very useful these days apart from determining who’s using Safari?
Edit: Not to mention that the onerror event handler on the <img> tag has always been able to find out if an image didn’t display [2].
For example, Firefox on macOS sent this Accept header for this request: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,/;q=0.8
Chrome sent text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,/;q=0.8,application/signed-exchange;v=b3;q=0.9