XHTML (eXtensible HyperText Markup Language) is a markup language that is a reformulation of HTML 4 as an XML application. This means that XHTML documents must adhere to the stricter syntax rules of XML, unlike traditional HTML which is often more forgiving of errors. The primary goal of XHTML was to make web content more modular, easier to process by non-browser applications, and to ensure better integration with other XML-based technologies. It enforces stricter rules such as requiring all tags to be properly nested, all tags to be closed, and all attribute values to be quoted. While it was intended to be the successor to HTML 4, modern web development has largely shifted towards HTML5, which incorporates many of the structural and validation benefits of XML while retaining the flexibility of HTML. However, XHTML remains relevant in contexts requiring strict XML compliance, such as mobile device rendering or specific data interchange scenarios where validation is critical. It is fundamentally a document format designed for structuring content on the World Wide Web.