Can we insert javascript into any webpage loaded in the browser

Try using Greasemonkey: http://www.greasespot.net/. You can use it to execute custom scripts on page load for any website you want. You can find some basic tutorials here: http://wiki.greasespot.net/Tutorials.

I suggest to create a page with two iframes one to navigate to the designated website and other to get DOM Objects. in the first one navigate to the site and then select its HTML and append it in the body of the second Iframe.

iframe2.contentWindow.document.body.innerHTML = iframe1.contentWindow.document.body.innerHTML

then traverse the DOM Objects inside the second Iframe with your custom functions