Files that contain the .xhtml file extension hold web pages that have been written using the XHTML syntax. The XHTML format is an improved version of HTML4, or rather, another way of writing web pages using a modified version of HTML.
The XHTML file format uses tags, just like the HTML file format, but the pages of the XHTML format conform to the stricter XML guidelines than HTML pages. Because the documents that are created in XHTML need to be well-formed, they are normally parsed using XML parsers, whereas HTML documents use more versatile HTML-specific parsers.
When a user visits a web page that is created in XHTML, they do not usually see the actual code that the page was written in. The user will see it through the web browser, and therefore usually don't need to care about what format the page is written in by the developer.
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If you have additional information about the XHTML file format or software that uses files with the XHTML suffix, please do get in touch - we would love hearing from you.