Here is what I want:

I go to https://www.secure.com, navigate to a specific page on the site, then save its content to a file. As long as the page is in the browser, the HTTPS protocol itself grants the authenticity of the page, but after saving it to a file, anyone can modify it. Let's say I don't trust myself and nobody trusts me, but I must prove that this page, that has been saved to disk, has original content generated by www.secure.com. I can't sign it with my own-generated private key because as I mentioned, nobody trusts me. They only trust www.secure.com. So I would like to save either a raw HTTPS content or a decrypted HTTP response with a digital signature that can be verified if someone takes a look at the (cert / public key / whatever) of www.secure.com.