Using TextEdit for HTML

Solution 1:

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.

Solution 2:

Use the ⇧⌘T key combination to render the document you're viewing as plaintext instead of WYSIWYG.