Open Text Edit.
Close the document.
>Text Edit Preferences.
1) Select Plain Text.

2) Select the Open and Save Tab.
Unclick Add ".txt" extension to plain text files.
Check Ignore rich text commands in HTML files
Check Ignore rich text commands in RTF files

Close the preferences window.
>File New
Save your document into your Mathography folder as

firstnameMathography.html.

Note. After you close the document and open it again, be sure to
Check Ignore Rich text Commands.