What Is a Markup Language? HTML is used to mark up the structure of Web pages, but other markup languages are used by computers on the Web and beyond. The Standard Generalized Markup Language (SGML) is the grandfather of most markup languages used for print and the Internet. SGML is the international standard used to define the structures and appearance of documents. Multiple SGMLs have been created for a variety of document types and for various specialties—such as physics, accounting, and chemistry. HTML and XHTML are the Web’s primary version of SGML.