Describe in detail what a metalanguage like XML is and what it can do.

What will be an ideal response?

The tags in HTML are, as we have seen, specified. The tag pair

, for example, is used for marking the enclosed text as a paragraph. The writer of the HTML document cannot invent new tags. XML (eXtensible Markup Language) is a newer markup language. It is a "metalanguage," that is, a markup language for markup languages. Using XML, the writer can create his or her own tags; an XML document is not about displaying information but about how to structure and interpret information to be displayed. An XML document usually also contains or refers to a schema that describes the data, and the body of the XML document can then be checked against the schema to be sure that it is a well-formed document. All modern browsers support mechanisms that translate XML documents into HTML documents for display. XML allows for flexible document interchange across the web; for example, in May 2003, the National Library of Medicine announced a "Tagset" for journal articles to provide a single format in which journal articles that originate from many different publishers and societies can be archived. As another example, LegalXML is a collection of XML standards for sharing and transmitting legal documents of various kinds, such as documents between courts and attorneys, court transcripts, and electronic contracts. More commonly, XML-based file formats now form the basis for office productivity tools such as Microsoft Office, Apple iWork, and Google Docs.