XHTMLiin

XHTMLiin is a hypothetical XML-based markup language that extends XHTML with an integrated semantic layer intended for durable web documents and rich client applications. It aims to marry familiar XHTML syntax with a lightweight framework for encoding data roles and relationships alongside presentation.

Like XHTML, XHTMLiin relies on XML rules, uses namespaces, and requires well-formed documents. It specifies an ii namespace for elements that carry semantic information and provides a schema to validate document structure. Authors write element names in lower case and quote attributes; documents can be served under traditional XHTML media types or a dedicated application/xhtmliin+xml type depending on deployment.

Core features of XHTMLiin include explicit semantic annotations through iin: attributes and elements, enhanced accessibility metadata, and a modular vocabulary designed to support progressive enhancement and offline caching. The language supports both a strict mode, in which all semantics are declared, and a permissive mode, which is intended to improve backward compatibility with standard XHTML content.

History and status: XHTMLiin originated in experimental standardization discussions in the late 2010s and remains a niche concept used mainly in research and hobbyist projects. It has limited browser support, and interoperability with existing XHTML tooling varies. Some implementations provide validators and converters to map XHTMLiin to plain XHTML or HTML.

Overall, XHTMLiin represents an exploratory approach to combining structure, semantics, and presentation within a single XML-based format, but it has not achieved widespread adoption.