Attach a link to an item, and Zotero stores only the page title, URL, and access date. To view the content, return to the site. Use this method when there is little chance of a Web page changing or disappearing. This method has the benefit of using negligible Zotero storage space.
Save a snapshot, a copy of the page as it currently exists, as an attachment to an item. Snapshots are a single web page (not a site), including links and images. A lot of snapshots can use a lot of Zotero storage space.
Prevent Zotero from automatically taking unnecessary snapshots of Web pages by unchecking the box in Preferences --> General-- "Automatically take snapshots when creating items from web pages."
When applicable, choose to intentionally take a snapshot of a specific Web page with a right-click/command-click on the Zotero browser connector and "Save to Zotero (Web page with snapshot)"
Perma is a service that makes an archival copy of a Web page and houses it in the Internet Archive. Taking a snapshot and storing it in Zotero might not be necessary.
Further, Perma provides a new public URL that can be added to your Zotero item and cited in publications, thus ensuring readers of your work access to the content of the Web page into the future.
Perma is free to Cornellians but requires registration.