Today, cacheable, unchanged resources are not stored in the browser cache - regardless of the Cache-Control response headers that are applied in Workers. In other words, it's impossible to get a 304 Not Modified response (it is, however, possible to get Chrome to return the resource from memory cache).

There are two reasons for this:

1. Resources stored in Cache API do not include a Last-Modified or Etag response header
2. The cacheKey tied to each resource would strip the If-Modified-Since or If-None-Match header before querying the cache API (right now, the browser wouldn't even send these headers because of the first reason)

Solutions
Coming soon