In order for SVG/SMIL to animate the "font" and "font-family" properties, we need nsStyleAnimation to support "font-family" values. As with enumerated values (in bug 520486), font-family won't need Add/ComputeDistance/Interpolate implementations. We basically just need a representation in nsStyleAnimation::Value, and code in ExtractComputedValue / UncomputeValue. Last I checked, I think "font-family" is represented in the style system as an enumerated value plus a possible string value (to handle non-standard family names).
Bug 529934 will actually fix this on the nsStyleAnimation side. I'm reshaping this bug into supporting "font-family" on the SVG/SMIL side.
Created attachment 413443 [details] [diff] [review] trivial fix