XML - Data notation

XML, aka Extensible Markup Language, is a data notation created in 1996.

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In computing, Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. The W3C's XML 1.0 Specification and several other related specifications—all of them free open standards—define XML. The design goals of XML emphasize simplicity, generality, and usability across the Internet. Read more on Wikipedia...

XML Wikipedia page
XML docs
XML specs
XML mailing list
There are at least 3,258 XML repos on GitHub
PLDB estimates there are currently 42k job openings for XML programmers.
file extensions for XML include xml, adml, admx, ant, axaml, axml, builds, ccproj, ccxml, clixml, cproject, cscfg, csdef, csl, csproj, ct, depproj, dita, ditamap, ditaval, dllconfig, dotsettings, filters, fsproj, fxml, glade, gml, gmx, grxml, gst, hzp, iml, ivy, jelly, jsproj, kml, launch, mdpolicy, mjml, mm, mod, mxml, natvis, ncl, ndproj, nproj, nuspec, odd, osm, pkgproj, pluginspec, proj, props, ps1xml, psc1, pt, qhelp, rdf, res, resx, rs, rss, sch, scxml, sfproj, shproj, srdf, storyboard, sublime-snippet, targets, tml, ts, tsx, ui, urdf, ux, vbproj, vcxproj, vsixmanifest, vssettings, vstemplate, vxml, wixproj, workflow, wsdl, wsf, wxi, wxl, wxs, x3d, xacro, xaml, xib, xlf, xliff, xmi, xmldist, xmp, xproj, xsd, xspec, xul, zcml, xsl and xslt
The Google BigQuery Public Dataset GitHub snapshot shows 2k users using XML in 3k repos on GitHub
XML on HOPL
ANTLR grammar for XML
XML LSP implementation
CodeMirror package for syntax highlighting XML
Monaco package for syntax highlighting XML
Pygments supports syntax highlighting for XML
GitHub supports syntax highlighting for XML
Indeed.com has 23,248 matches for "xml developer".
See also: (22 related languages)SGML, Unicode, SOAP, Office Open XML, UTF-8, ASCII, HTML, Regular Expressions, XPath, XQuery, Scala, Java, Smalltalk, PHP, Python, ECMAScript, RDF, JavaScript, HyTime, JSON, YAML, S-expressions

Example from hello-world:

<?xml version="1.0" encoding="UTF-8"?>
<text><![CDATA[Hello World]]></text>

Example from Linguist:

<?xml version="1.0"?>
<gml:Point xmlns:gml="http://www.opengis.net/gml" srsName="urn:ogc:def:crs:EPSG::4326" gml:id="uuid.12b3c8bb-bc8a-4f83-9085-1a5f3280b8ba">
  <gml:pos>52.56 13.29</gml:pos>
</gml:Point>

Example from Wikipedia:

<?xml version="1.0" encoding="ISO-8859-1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"></xs:schema>