Implement the CSS Font Loading specification. There is already an older implementation in place. The new work is to update it to the current spec and enable it.
The spec has changed radically since the initial implementation work was done. Specifically, the Font Loading API is now Promise-based, so the update will be a non-trivial amount of work.
We (Adobe Typekit) are using this API quite heavily in our "kits" to detect font load events. If this API were supported by WebKit, we could avoid the expensive fallback strategy of polling a element for its width every N milliseconds.
It would be great if this feature received some attention.
For reference, I've described our use of the API in more detail here:
Assigning to myself.
*** This bug has been marked as a duplicate of bug 153346 ***