Difference between document.body.contentEditable='true'; and document.designMode='on';

In this case, they're equivalent. contentEditable makes the element it's applied to and all descendant nodes editable. designMode makes the entire document editable.

There's a great summary of the history of both properties available on the WHATWG blog — The Road to HTML 5: contentEditable. It's worth a read, if you're interested.

contentEditable allows to make specific elements editable while designMode affects the whole document.