The in-world webpage on a face is in-fact client side. The only parts that are streamed are actions done to the page, thus why when you type in a input field it slowly updates on other clients which are also viewing the same prim. If you haven't tried, create a script in php to dump $_SERVER vars and have different users compare the REMOTE_ADDRESS that shows up. They will be different and it will be your IP. This thus proves that it is possible to send client specific variables over each HTTP header, at the discretion of the avatar through some settings panel. Also avatars can override the autoloading feature for webpages. I say, send object as well as agent data over headers, just like a standard LSL http_request sends headers including names, keys, and locations.