Never Used Mac Os But..
I am sure the issue you are running into is that once you save a file as .hmtl from the text edit app, the file is no longer using the text edit standard file extension. A program only knows what files that it is able open and use by the letters following the filename, like .txt .doc .exe These are examples of file extensions. This idea is called "file association".
In windows you can right click on a file and tell it to "open with" and choose a program, I doubt this is an option for you. However you can simply open the text edit app and then choose file>open and then locate the html file to open.
Or you can rename the file to .txt and open it, then re save as .html
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