Expose information about how a resource was delivered. For example, resources which were delivered from the cache (currently exposed through transferSize) and navigations which were prefetched by the previous page are useful to identify.
This provides a way for navigational prefetch to be detected by authors, and additionally provides a more natural way to mark resources served from the HTTP cache. Potentially also useful for marking resources served from the preload (as in <link rel=preload>) cache.
Explainers: https://gist.github.com/jeremyroman/43f8f290f1f404d3b7f6cb708601c7f0