Wikipedia
DHTMLEdit is an ActiveX control for Microsoft Internet Explorer 5 and later. It allows Web authors and application developers to add WYSIWYG DHTML editing capabilities to their Web sites and applications. The editing component uses Microsoft's Component Object Model (COM) technology to provide access to editing services such as basic HTML formatting, tables, undo and redo, and absolute positioning.
Because of several security issues with this ActiveX control, Microsoft decided to discontinue shipping it with Windows Vista. Also, Internet Explorer has continued to evolve, incorporating WYSIWYG editing features of its own in Internet Explorer 5.5 and Internet Explorer 6. The control was present in the Beta 1 version of Windows Vista, but is absent in subsequent Windows releases.
However, because several programs and websites require this control to function correctly, it is still available as a separate download from Microsoft Download Center. More information can be found here.