I keep the URL as the original source so I know where I originally found it. Then I extract some unusual phrases and keep them verbatim so I go search later if the original goes missing.
Sometimes I then curl the original, and sometimes I "lynx -dump" it so I can have the original plain text.
You say:
> I use the Zettelkasten method ...
This means different things to different people. Many people read about the Zettelkasten and think it's basically a collection of "index cards", each with a single thought on it. As such it would be no different from a wiki with lots and lots of small pages. My understanding is that a Zettelkasten is much, much more than that. My understanding is still evolving, and I'd love to hear more about what you actually do.
Have you written it up? Would you do so and post a link? And email me in case I don't see it?