I recommend you have a rethink about what you want to achieve. Unfortunately, Microsoft Word is close to the most awful html editor ever devised. It 'can' save a document in html format but that content is prodigiously complex and poorly handled by the multitude of web-browsers that might be used to display it.

Web design has moved a long way since the days of frames and subpages. Nowdays page layouts need to work on a huge range of browsers and devices (like mobile phones, ipads, chromebooks, laptops, smart TVs etc). You can find great resources online to help you learn how to code web pages from scratch. For instance this video EASY! Hand-code an HTML + CSS layout - YouTube shows the basic structure.
This video shows how to fix the header position to stop it from scrolling off the screen. How to make a Fixed Navigation Bar using HTML CSS | Collapsing Header Tutorial - YouTube

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Andrew Lockton
Chrysalis Design, Melbourne Australia