Using HTTP cache control for invalidating a cache of parsed objects

Do we have infrastructure in place that allows a cache on a higher
layer than Necko maintain cache coherence according to HTTP rules when
the higher layer maintains a cache of object that have been parsed
from HTTP responses so that if the cache is fresh (from the HTTP point
of view) the objects don't get reparsed?

Does the image cache use something like this by any chance?

Should a higher-level cache even want to maintain HTTP-correct cache
coherence in situations where determining cache coherence would
require pinging the origin server with an etag and should a
higher-level cache work with expiry times instead and simply not
support etags?

(I'm thinking of a localization resource cache for a situation where
L20n grows past chrome: URLs to work with HTTP-originating
localization resources that are shared between documents within an
origin.)

-- 
Henri Sivonen
[email protected]
http://hsivonen.iki.fi/