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HTML or HyperText Markup Language is the main markup language for creating web pages and other information that can be displayed in a web browser.and XHTML (Extensible HyperText Markup Language) is a family of XML markup languages that mirror or extend versions of the widely used Hypertext Markup Language (HTML), the language in which web pages are written.
HTML is based on SGML (Standard Generalized Markup Language), while XHTML is based on XML (Extensible Markup Language) , a more restrictive subset of SGML. XHTML documents are well-formed and may therefore be parsed using standard XML parsers, unlike HTML, which requires a lenient HTML-specific parser. In particular XHTML requires all tags to be closed - <br />, <hr />, <img />. While HTML permits "empty" tags - <br>, <hr>, <img>.