Document.designMode

document.designMode controls whether the entire document is editable. Valid values are "on" and "off". According to the specification, this property is meant to default to "off". Firefox follows this standard. The earlier versions of Chrome and IE default to "inherit". Starting in Chrome 43, the default is "off" and "inherit" is no longer supported. In IE6-10, the value is capitalized.

var mode = document.designMode;
document.designMode = value;

A string indicating whether designMode is (or should be) set to on or off. Valid values are on and off.

Make an <iframe>'s document editable:

iframeNode.contentDocument.designMode = "on";

• Rich-Text Editing in Mozilla
• HTMLElement.contentEditable