forensics – Preserving crime evidence from a website I do not own

Is there a standard procedure when it comes to preserving evidence of a crime committed on a website of a third party?

Let’s assume that a crime was committed on a website I visit, like defamation. Other than using the Wayback Machine or browser plugins to save the page in case either the website owner or the perpetrator removes the evidence, is there a better way?

My goal is to be able to prove to the authorities that a crime was committed so that they start an investigation. For not-so-serious crimes unless it’s very easy to prosecute – e.g. the evidence is still accessible without the need for a warrant – the police might not even bother.