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 HTML which is more forgiving. The primary goal of XHTML was to bridge the gap between HTML and XML, making web content more structured, machine-readable, and easier to integrate with other XML-based technologies. Because it is an XML application, XHTML requires all tags to be properly nested, all tags to be closed, and attribute values to be quoted. It was intended to be the successor to HTML 4.01, offering better integration with future web standards and devices. While modern web development primarily relies on HTML5, which incorporates many of the structural benefits of XML while retaining some of HTML's flexibility, XHTML remains relevant in specific contexts, particularly where strict XML compliance is required, such as in certain mobile device rendering environments or when integrating web content into XML data streams. It is essentially HTML written according to XML rules.