It'd be nice (and theoretically probably easy) to allow @font-face to fall back with multiple src:s.

@font-face {
  font-family: sans-serif;
  src: url("/usr/X11R6/share/fonts/ttmn.ttf");
  src: local("TT MN");
}

Where the first src: that works is the src: that is used. It might be more "CSS"-like if they were combined into one line, e.g.,

@font-face {
  font-family: sans-serif;
  src: url("/usr/X11R6/share/fonts/ttmn.ttf") local("TT MN");
}

This already works, but you need to separate them with commas, just like in the font-family property.