Method to prove authenticity of download files in hindsight

I'm looking for a tool or method to prove the authenticity of resources download from the web and stored locally. To be clear: I don't mean the SHA or MD5 checksums to verify a downloaded file. What I need is a way to download and store a web resource in such a way that I can later prove that said resource indeed originated from that web server.

In particular for the following scenario: A website published an article about a client. He would like to sue for defamation of character. I need a way to store the article without them having the possibility of simply removing it and denying they ever published it. So preferably this would be a tool that is backed by publications making it credible in court.

I have thought about storing the TLS certificate, keys and the encrypted data. That would rely on the root CA, but I think that would in itself not be a problem. I could do this using a custom program and a library like OpenSSL, but I think this is such a common problem, there probably is a relatively standard tool for it. Also, I am not entirely sure to what extent this would constitute reliable evidence. And can someone point to publications that would back this method?

Maybe I am using the wrong search terms, but everything I find is about aforementioned SHA or MD5 checksums. Any help is much appreciated.