browser script to read webpages before displaying them

[hopsalo]

Might seem a strange question but I have a brain fart...is there a cross-browser way to change browser behavior?What I want to do is make say a JavaScript script to be run by the browser before it loads a page (when browsing the internet for example). I would want everything to be read and stored by the script and then the script pass it on as a var (for example) to the browser for display. It will always be reading the page and passing it on to the browser, acting as a middle man doing what I want to the page or looking for malicious behavior...Has this been done before or at all possible?Thanks!