Editing Markup language (section)

===XML===
{{Main|XML}}

XML (Extensible Markup Language) is a meta markup language that is very widely used. XML was developed by the [[World Wide Web Consortium]] in a committee created and chaired by [[Jon Bosak]]. The main purpose of XML was to simplify SGML by focusing on a particular problem — documents on the Internet.<ref>{{cite web |url=https://www.w3.org/TR/xml11/ |date=16 August 2006 |title=Extensible Markup Language (XML) 1.1 (Second Edition) |publisher=W3C |access-date=2021-08-16 |archive-date=2021-08-11 |archive-url=https://web.archive.org/web/20210811223058/https://www.w3.org/TR/xml11/ |url-status=live }}</ref> XML remains a metalanguage like SGML, allowing users to create any tags needed (hence "extensible") and then describing those tags and their permitted uses.

XML adoption was helped because every XML document can be written in such a way that it is also an SGML document, and existing SGML users and software could switch to XML fairly easily. However, XML eliminated many of the more complex features of SGML to simplify implementation environments such as documents and publications. It appeared to strike a happy medium between simplicity and flexibility, as well as supporting very robust schema definition and validation tools, and was rapidly adopted for many other uses. XML is now widely used for communicating [[database transaction|data]] between applications, for serializing program data, for hardware communications protocols, vector graphics, and many other uses as well as documents.

====XHTML====
{{Main|XHTML}}
From January 2000 until HTML 5 was released, all [[W3C Recommendation]]s for HTML have been based on XML, using the abbreviation [[XHTML]] ('''Ex'''tensible '''H'''yper'''T'''ext '''M'''arkup '''L'''anguage). The language specification requires that XHTML Web documents be ''well-formed'' XML documents. This allows for more rigorous and robust documents, by avoiding many syntax errors which historically led to incompatible browser behaviors, while still using document components that are familiar with HTML.

One of the most noticeable differences between HTML and XHTML is the rule that ''all tags must be closed'': empty HTML tags such as <code><nowiki><br></nowiki></code> must either be ''closed'' with a regular end-tag, or replaced by a special form: {{nowrap|<code><nowiki><br /></nowiki></code>}} (the space before the '<code><nowiki>/</nowiki></code>' on the end tag is optional, but frequently used because it enables some pre-XML Web browsers, and SGML parsers, to accept the tag). Another difference is that all [[HTML#Attributes|attribute]] values in tags must be quoted. Both these differences are commonly criticized as verbose but also praised because they make it far easier to detect, localize, and repair errors. Finally, all tag and attribute names within the XHTML namespace must be lowercase to be valid. HTML, on the other hand, was case-insensitive.

====Other XML-based applications====
Many XML-based applications now exist, including the [[Resource Description Framework]] as [[RDF/XML]], [[XForms]], [[DocBook]], [[Simple Object Access Protocol|SOAP]], and the [[Web Ontology Language]] (OWL). For a partial list of these, see [[List of XML markup languages]].