[css-font-loading] What base url do we use for FontFace()?

[tabatkins]

The FontFace constructor can take a URL string (technically, something that matches the grammar of @font-face/src), and will load the font from that URL. In CSS, relative URLs are resolved against the stylesheet's own URL; in this case, tho, there's no stylesheet to resolve against. What base url should we use?

(Presumably implementations have already made some decision; if someone knows what that is, great, otherwise I'll test for this later.)