I say ironically: What a great hint! One can change the content of any page to something obnoxous than print it an 'prove' that the original page owner did this dastardly thing.

Obviously I find that one can even do this highly objectionable and a really bad 'feature' of a browser.

But you have always had the possibility to save a web page, edit it, open the edited page in a browser and printed it.

And screenshots are easy to manipulate in image editing software, in fact that is what many webdesigners do on a regular basis. They take an image of an existing web page and show how they could improve on it for the customer.

If you take a printout or screenshot for "evidence", then you really need to learn what to trust.