This is not a CSSStyleDeclaration; the actual set of supported properties needs to be different. So it's not at all clear to me that the named property thing should work at all.
That's not what the spec says... <http://dev.w3.org/csswg/cssom/#cssfontfacerule>
Well, the spec is totally broken. What, exactly, should .style.azimuth return on a font-face rule? What should happen when you set .style.display on such a rule? Maybe this section of the spec was written before CSS2Properties was merged into CSSStyleDeclaration? Or maybe like most of CSSOM it's just broken and unmaintained? Do you want to raise the spec issue, or should I? ;)
Created attachment 637268 [details] [diff] [review] Make CSSFontFaceStyleDecl an nsICSSDeclaration.
Note that in bug 753517 I plan to disappear these props from here altogether (which will be somewhat different than them throwing). Hence the lack of a test for them working, though I could certainly add a test for the getter not throwing.
Comment on attachment 637268 [details] [diff] [review] Make CSSFontFaceStyleDecl an nsICSSDeclaration. r=dbaron