It looks like font loading events' interface has been chained in the latest spec.
The latest interface is FontFaceSet: http://dev.w3.org/csswg/css-font-loading/#FontFaceSet-interface
There's one more issue with the current implementation, which is that it prevents many documents to go into a PageCache because CSS fonts are large and they continue to load for a while. I think we need to come up with a strategy to suspend the font loading manager somehow.