Per the discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=1709415 it seems -webkit-image-set may be used enough on the wild that supporting it as a parse-time alias would be a low effort compat win.

If my reading of the WebKit code is right, WebKit supports the prefixed version with an slightly stricter syntax, but they serialize to the modern syntax. I think it'd be good if they just supported the same syntax, since the modern syntax is an extension of the webkit one, and it's less maintenance.

cc @smfr.