document.body.contentEditable = true
There are actually easier ways to do the same thing: First, you can just paste
But, wait, there's more: The hint from canisbos was actually inspired by an earlier hint from rab777hp.That tip requires you to enable Safari's ever-helpful Develop menu, too. Then, when you see a snippet of text that you’d like to modify, right-click on it and choose Inspect Element from the contextual menu. (If the text you want to edit isn’t immediately apparent in the resulting Web Inspector panel, use the search box at the upper right.) Once you find the text you want to tweak, right-click on it, and choose Edit Text. Type away to your heart’s content, and then hit return. The Web page you’re modifying will update instantly.
And sorry, would-be defacers: Any tweaks you make will be visible to you alone (until you refresh); they won’t affect what anyone else sees when they visit that page.