The usual browser conditional request looks like this:
HTTP/1.1 Host: us.yimg.com
If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT
If-None-Match: "10c24bc-4ab-457e1c1f" HTTP/1.1
But what is the relation between If-Modified-Since and If-None-Match. OR? AND? So if any of them "fails", a new version will be sent?