A markup language is a set of tags that describe the content of a document. The most common markup languages today is a markup language is not a programming language, hence it is a misnomer to 'code' HTML. has introduced extensibility into HTML called . This new more versatile language can incorporate many types of media, other than plain text; and web browsers are constantly adding more support for languages such as and . XML has strict rules that can increase the learning curve for a person new to XML-based markup. However once the rules are understood, it leads to more consistent and error-free markup which allows more devices to process the information correctly.