The design mode lets you edit the content of any website directly inline in a WYSIWYG-like fashion. The modifications are locally only but still useful if you want to test something. Keyboard shortcuts like Ctrl+B for bold are supported.

The design mode can be enabled with one line of JavaScript:

// Enable
document.designMode = "on";

// Disable
document.designMode = "off";

// Toggle
document.designMode = document.designMode == "on" ? "off" : "on";