When a request is sent to a Web site, I'd like to edit some code to the returned page and the edited page is loaded to the browser. For example, when I send a request to statckoverflow.com, I'd like to change all the letter to upper case in the returned Web page, and then the edited page is loaded to my browser. (It's just an example to change the case.)

I'm thinking to use a proxy on my machine. Is there any implemented proxy that can do this? Or is there any other way?

The edition operation is going to be complicated. So, the proxy should be able to have some scripts deployed.