What is the relationship between SGML, HTML, XML, and…

SGML (Standard Generalized Markup Language) is a meta-markup language, an ISO 8879 standard. It defines the syntax for describing other markup languages, not a markup language for documents itself. HTML and XML are applications (definitions) of SGML, but with different purposes and restrictions.

HTML (HyperText Markup Language) is a markup language based on SGML, designed for creating web pages. It has a fixed set of tags and rules for their use, with more flexible syntax rules compared to XML (for example, it does not require all tags to be closed).

XML (eXtensible Markup Language) is another meta-markup language, a descendant of SGML, but significantly simpler and stricter. The main difference from SGML is that XML focuses on transmitting structured data, not document presentation. Unlike HTML, XML does not have a fixed set of tags — they are defined by the user. It requires strict adherence to syntax (always closing tags, proper nesting, etc.).

XHTML (eXtensible HyperText Markup Language) is a reformulation of HTML 4 as an XML application. That is, it is HTML but with stricter syntax, similar to XML rules. It was an attempt to combine the advantages of HTML (for web pages) and XML (strictness, extensibility), but today it has been largely replaced by HTML5.

Thus, the relationship is as follows:

SGML is the parent/base for HTML and XML.
HTML and XML are applications of SGML (having syntax defined by SGML rules).
XHTML is a version of HTML rewritten with XML syntax rules.
HTML is created for displaying documents in browsers, XML for transmitting and storing structured data.
XHTML tried to combine display (HTML) with strictness (XML).

In short, the dependency chain can be represented as: SGML -> HTML, SGML -> XML. And also: HTML + XML syntax -> XHTML.

Example structures:

<!-- Example HTML -->
<!DOCTYPE html>
<html>
<head>
    <title>Title</title>
</head>
<body>
    <h1>Greeting</h1>
    <p>This is a paragraph.</p>
</body>
</html>

<!-- Example XML -->
<book>
    <title>My Book</title>
    <author>Ivan Ivanov</author>
    <genre>Science Fiction</genre>
</book>

<!-- Example XHTML -->
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
    <title>Title</title>
</head>
<body>
    <h1>Greeting</h1>
    <p>This is a paragraph.</p>
</body>
</html>

Note the strict closing of tags in XML and XHTML compared to HTML.

What is the relationship between SGML, HTML, XML, and XHTML?