History, a panel on a site's owner page, keeps every change to the site for a year, so any of them can be undone.

It lists the latest 15 changes, newest first, and What changed opens each one. Undo beside a change puts the site back as it was before it, which takes back every change made since as well.

While you type, ⌘Z undoes the last edit, and a change made from the ask bar can be undone at once from the Undo beside Done.

A wiki keeps a history for each page instead; see Write a wiki.