Yes you can: you can take a web snapshot. A web snapshot allows you to retain the presentation of a particular web page at any time, even if the original is later altered or removed. After browsing to the web page you want to preserve, click on the camera icon in the middle of your Zotero pane. A new web snapshot item will appear in the item list. Click on View Snapshot on the right-hand side to view the file. You can drag the snapshot over an existing item to make it a child of that item in the center column.