I haven’t actually done anything with font-face
, so take this with a pinch of salt, but I don’t think there’s any way for the browser to definitively tell if a given web font installed on a user’s machine or not.
The user could, for example, have a different font with the same name installed on their machine. The only way to definitively tell would be to compare the font files to see if they’re identical. And the browser couldn’t do that without downloading your web font first.
Does Firefox download the font when you actually use it in a font
declaration? (e.g. h1 { font: 'DejaVu Serif';
)?