Support for basic Cache-Control headers was added with #192 / #261. For more complete header support, the next logical step would be handling ETag and Last-Modified headers.

Here is an outline of how I would expect that to work:

• When a cached response is retrieved and it's expired:
  • If it contains an ETag header, add an If-None-Match header to the request
  • If it contains a Last-Modified header, add an If-Modified-Since header to the request
  • If a 304 Not Modified response is received, returned the cached response
  • If a new 200 response is received, update the cache and return the new response

Other related headers to consider:

• If-Match
• If-Unmodified-Since
• If-Range

I'm unsure if/how those should be used within requests-cache, though.

If this is a feature you would like, please comment!