CSSFontSelector::addFontFaceRule decides whether to create a SVGCSSFontFace or CSSFontFace based on the first item in the source list. If that item is not an in-document font-face element, then a CSSFontFace is created, which does not support font-face elements at all. See //FIXME in that function.
*** This bug has been marked as a duplicate of 16880 ***