Restoring the default HTML viewer for Internet Explorer

if you have added a custom HTML viewer for Internet Explorer and would like a simple way to revert to the original built-in viewer, it doesn’t appear immediately obvious how to do this in IE.

The simple fix is edit registry, using Regedit, to remove this whole registry entry here:

Just right click the key :

Computer\HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\View Source Editor

Click Delete and then click the Yes button in the confirm dialogue.