Sorry, still don't understand. The dom is not complete until JavaScript has run, no? So the ublock extension can do dom traversal on the rendered page, but the proxy can only operate at the request level. I assume your not running a headless browser on your squid box, lol. I'm aware that a proxy can tweak html and js, but the js will be challenging to analyse statically. If you're only blocking requests to ad domains, then presumably most people would just block them in DNS. So i still don't understand what the proxy gains you.