You want to test out new font colours, or new patterns or may be test out some other blogs design and see how it would look for your content, then either you need to own the other website or else need access to it. If you have no options then there is one option where in you need to post some code which is Java based on your Address Bar and then you can edit any web page on the Internet.
You wont be able to save any of these web pages because this is a option which can just let you to edit any website pages which are shown on your browser and nothing else. In order to show you we just entered the following code on our address bar after loading the website TechCrunch.com on the browser.
The following screen-shot shows you a small demo of the stuff we are talking about.