This is very, very awesome work. My team has been working on enabling the PRPL pattern[1] and differential serving on a few platforms, and edge caching has been a problem. We've tried to use Vary: User-Agent, but that leads to low cache hits. This API would let us to much smarter UA sniffing at the edge.
From there we just need to parse some responses like JS and HTML, to be able to push their sub-resources, for an instant speedup and great caching for fine-grained deployments.
[1]: https://developers.google.com/web/fundamentals/performance/p...