Content based caching would be a massive security nightmare. All you'd need to do is serve up the same asset as any other site on the web and you could instant

Content based caching would be a massive security nightmare. All you'd need to do is serve up the same asset as any other site on the web and you could instantly know if the user has been there recently. No need for tracking at all, just serve this up to people who go to HN, this to the redditors, this to anyone that was recently on 4chan... Or let's take it a step further. I could reasonably figure out what your user-page on HN looks like to you when you're logged in. I'll serve that up to all my visitors and when I get a cache-hit I know it's you!