XML Is Variant of SGML not a subset[]
Speaking technically, XML is a "Strict variant" of SGML. Several formatting functions are tightened to ensure strict block formatting, better handling of data typing and better handling of scripting integration. SGML does not have this strict nature, namely because it predates the scripting integration features found in HTML 2 and HTML 3 that slowly made it necessary to implement the XML standard draft in HTML4 strict and in HTML5. Both languages support "freeform" variable assignment, and a wide variety of data types. However, XML uses strict typing and strict block form, while SGML utilizes loose. Putting a number into a string on SGML works, but on XML it doesn't. Likewise, Non-Block tags (such as HR in HTML) require the ending "/" in XML, but it is optional in SGML. (It is even unsupported in early SGML versions.) HTML1-3, VRML, and HTML4 loose are examples of subsets of SGML. HTML4 Strict, XHTML, HTML5, X3D, RSS, RDF, RTF and SVG are examples of XML formats. Because XML is compliant with HTML but not visa-versa, it's usually a good idea to just be XML compliant when writing your code. (Proper usage of quotes, parenthasies, closing "/" marks and other block syntax insures XML compliance). However, this does not work for other people's codes, so make sure you import using SGML compliant tools. --Ruedii (talk) 23:42, 5 December 2019 (UTC)