Using TextEdit for HTML
It is in TextEdit help - search for HTML


  If you open an HTML file and don’t see the HTML code, TextEdit is displaying the file in the same way a browser would. You can choose to display the file’s HTML code instead.
  
  To display the code in an HTML file:
  
  Choose File > Open.
  
  Select “Ignore rich text commands.”
  
  Locate the file and click Open.
  
  To always view the code in HTML files,
  choose TextEdit > Preferences, click
  “Open and Save,” and then select
  “Ignore rich text commands in HTML
  files” below “When Opening a File.”


So it seems you have to choose before opening the file.