XHTML
XHTML (Extensible HyperText Markup Language) is a markup language that combines the syntax of XML with the semantics of HTML, designed to be a stricter and cleaner version of HTML. It enforces well-formedness rules, such as requiring all tags to be properly closed and nested, making documents more compatible with XML parsers and tools. XHTML was developed to bridge the gap between HTML and XML, enabling web content to be processed by a wider range of applications.
Developers should learn XHTML when working on projects that require strict adherence to markup standards, such as in enterprise environments or for content that needs to be parsed by XML-based systems. It is particularly useful for creating web pages that must be interoperable with other XML technologies, like RSS feeds or SOAP web services, and for ensuring cross-browser compatibility in legacy systems. However, its use has declined in favor of HTML5, which offers more flexibility and modern features.