Understanding Markup Languages: HTML vs XML

Both XML and HTML are markup languages. HTML is designed for displaying data with focus on how data looks. XML is created to store and transport data, with focus on data's meaning.

HTML structures web page content. It uses predefined tags to render text, images, and interactive forms. HTML5 is the latest version which supports multimedia elements without requiring additional plugins.

XML allows users to define their own tags. Its primary purpose is to facilitate data sharing across systems. XML is both human- and machine-readable, and is used in web services like SOAP and REST.

HTML syntax is more lenient with errors, while XML is strict, requiring all elements to be correctly nested, closed, and case-sensitive. Well-formed XML documents are crucial for parsing.

HTML focuses on how content is displayed and interacts with users. XML, on the other hand, is concerned with transporting data and its context, often used as a data feed for other applications.

XML is fully extensible. Users can create custom tags to define their data's structure and semantics. HTML is not extensible; it uses a standard set of tags defined by the W3C.

While HTML is the backbone of web page design, XML often works behind the scenes in APIs, configuration files, and is integral to technologies like AJAX for web app development.