A file with .xht extension is a web file associated with the XHTML (Extended Hypertext Markup Language) file type, which itself is an XML based markup file. Both these files are similar to HTML but have a more stringent XML-like syntax. XHT files are designed in a way to be more structured, involves less scripting, and are generic in addition to using the existing facilities of XML.
XHT File Format
An XHT file is created and stored as a plain text file with UTF-8 encoding by default. It contains the complete source code of an XHTML document that can be opened and edited with any text editor that supports Unicode encoding. In addition to text editors, XHT file format is understandable by most of the modern web browsers and can be opened using these.
The following example of an XHT document defines the XML declarations.
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
<html xmlns#"http://www.w3.org/1999/xhtml" xml:lang#"en" lang#"en">
<p>changed to <a href#"http://sample.com/">sample.com</a>.</p>