Xht

Xht is a historical file extension used for XHTML documents. In the early 2000s, as XHTML emerged as an XML-based reformulation of HTML 4.01, some authors saved their files with the .xht extension instead of .xhtml. The extension appeared in editor defaults and on servers that recognized it as XHTML source, but actual interpretation depended on the MIME type served, not solely the extension.

XHTML combines HTML’s element set with XML syntax, requiring well-formed documents: all elements must be closed, elements properly nested, and attribute values quoted. Documents often begin with a DOCTYPE declaration and may include an XML prolog. When served as application/xhtml+xml, browsers parse the document as XML and enforce well-formedness. If served as text/html, a browser typically treats it as HTML, potentially bypassing XML constraints and losing XML advantages.

Today the .xht extension is rare. XHTML support has declined in favor of HTML5, which uses the more common .html extension and does not require XML syntax. Some legacy projects or tools continue to use .xht or .xhtml for XHTML content, but modern deployment relies more on MIME type configuration than on file extension.

In summary, xht denotes a former convention for naming XHTML documents, reflecting a transitional period in web standards when XML-based HTML was experimented with, before HTML5 became the prevailing standard.