Designed to replace HTML Version 4.0, XHTML is recommended by the World Wide Web Consortium. XHTML Version 1.0, also known as HTML 4.01, is a reformulation of HTML 4.0 as an XML 1.0 application.

As said before, you need to use a third-party program to convert text to HTML on Windows 11 or Windows 10 PC. Although you can change the file extension to get it done, it is not recommended at all.