Viewing a web page's source in Safari:

 From Safari menu, select Develop > Show Page Source.
 If the Develop menu is not visible, go into Settings in the Advanced section 
 and select Show Develop menu in menu bar.

 Or, right-click on page and Show Page Source from drop-down menu.

 Or, Keyboard shortcut: Option+Command+U.

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TextEdit by default renders an .html file, like a browser.
So to view and edit the actual HTML content of a .html file
you need to 
  Settings | Open... tab |  checkbox about HTML
then close and re-open the .html file, you should now see the HTML of it,
not the rendering of it.

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TextEdit's default is to make a RTF file (a simple word processor)
so if you want to create and edit a plain text file, such as a HTML file:
 Create a new file in TextEdit, Save it. Exit.
 Rename the file to have extension .html (or .txt)
 Edit it. Delete the RTF junk.