Do I Need To Be Connected
... to the Internet to create HTML Pages?
No, in fact you don't need to be connected to the Internet at all to create web-pages. If the pages are not ready for publication or the site is not ready ... the pages can be stored on your computer, a ZIP drive, a flash drive, wherever.
I have spent months generating entire web-sites offline prior to "publishing" them online.
Using a Macintosh Computer
If you are using TextEdit on a Macintosh computer, the steps for creating a web-page are a little different than when using Windows Notepad.
From the Format Menu, you must first select Make Plain Text and then change the preferences under the Saving header by unchecking the box for Append '.txt' Extension to Plain Text Files.
Also, the default preferences are set to show .html as they would appear in a browser, which will not allow you to edit them. To fix this, check the Ignore Rich Text Commands in HTML Files under the Rich Text Processing header.