Boomla comes with built-in undo/redo functionality. Note that it records the state of the website’s filesystem, not keypresses when editing a text. When editing a text, the state is added to the undo list once you press the save button in the toolbar.
Currently, the undo history is 50 long. This may change.
Anything you do within the website can be undone. Also, it is pretty fast. Enjoy.