When saving a web page to disk as a web archive, the source URL should be stored in the file metadata, just as it is for downloaded PDFs.
The source URL in the metadata (displayed in the Finder’s Info panel) would allow the user to see where the archived web page came from, just as with downloaded PDFs. Among other things, they might want to compare the current version of the web page with the archived one. Or, they might need the URL to provide the source of a quote. This is not different from the PDF use case.