[somebodythere]

There could be an extension to the model, where a quoter can provide a snippet to the author's website and request a signature (perhaps from a common path like /.well-known/quotesign.)

If the snippet does indeed come from the website in question, the website will return a signature which the quoter can embed along with the snippet on their own website.

The signature would prove that the content came from the same person who controls domain X (as attested by the CA fo your choice). The user agent can display all this information where the content is quoted.