The following flags control the frequency of cached content validation when neither LOAD_BYPASS_CACHE or LOAD_FROM_CACHE are set. By default, cached content is automatically validated if necessary before reuse.
VALIDATE_ALWAYS forces validation of any cached content independent of its expiration time.
VALIDATE_NEVER disables validation of expired content.
VALIDATE_ONCE_PER_SESSION disables validation of expired content, provided it has already been validated (at least once) since the start of this session.
NOTE TO IMPLEMENTORS: These flags are intended for normal browsing, and they should therefore not apply to content that must be validated before each use. Consider, for example, a HTTP response with a "Cache-control: no-cache" header. According to RFC2616, this response must be validated before it can be taken from a cache. Breaking this requirement could result in incorrect and potentially undesirable side-effects.