contentEditable is an HTML attribute (invented by Microsoft and standardized in HTML5) that is used for client-side in-browser "rich text" editing.
There are really two attributes involved, designMode and contentEditable. The designMode attribute governs the entire document (i.e. it makes the entire document editable, like a dedicated HTML editor). The contentEditable attribute governs just the element on which it appears, and that element's children -- like a rich text editor control within a page.