XHTML is an XML-based HTML file format with its own version control to prevent confusion with the previous HTML.
Initially, and up to HTML version 4.x, HTML was defined with the help of the Standard Generalized Markup Language (SGML). At the same time, however, XML format became increasingly more significant, especially for the Internet. For this reason, it was attempted to use XML to define HTML as well and no longer use SGML for this purpose.
The result was the XHTML (Extensible HyperText Markup Language) file format. Across provides a separate template for XHTML files, which can be accessed via the drop-down list under Tools System Settings > Document Settings > Visual XML in the Document settings template section.