When you click the View Source command in Internet Explorer, it opens the html source using IE (F12) Developer Tools. If you want to change the default view source editor to a different viewer such as Notepad, Notepad++, use these steps:
Changing the View Source Editor in Internet Explorer
1. Launch the Registry Editor (Regedit.exe) and navigate to:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Internet Explorer
2. Create a subkey named View Source Editor
3. Under View Source Editor, create a subkey named Editor Name
4. Select the following key:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Internet Explorer\View Source Editor\Editor Name
5. Double-click (default) and set Notepad.exe as its data.
Note: If you prefer a third-party text editor (such as Notepad+, Notepad Lite etc), type the full path to the executable. For example, I’ve set Notepad++ as the default view source editor.

6. Exit the Registry Editor.
Article was last updated on August 16, 2016. Information in this post applies to all versions of Windows, including 10 Anniversary Update, aka Redstone 1 (Version 1607.)
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