If he can only associate my previous page view with my current requests, then how does he know it's me again? I mean, the image request is separate from the page request, and in any case only comes later.

So he would need to store a mapping from something he already knows (from the headers of my request for his html page, or my IP) to my ETag "cookie" to know what my previous ETag was.

Wouldn't that require using some of the features he wasn't going to use (like user agent) to work?

What am I missing?