In web standards, HTML is an application of which earlier, more robust document markup language?

Introduction / Context:
HTML did not appear in isolation. It was originally defined as an application (that is, a concrete markup vocabulary) of a more general and powerful standard for describing documents. Knowing that parent standard clarifies why HTML looks the way it does and how the web evolved toward XML and HTML5.

Given Data / Assumptions:

• The question asks which prior markup language family HTML descends from.
• Options include XHTML, XML, and SGML, plus a distractor.
• We focus on historical standards, not modern implementations.

Concept / Approach:
SGML (Standard Generalized Markup Language) is an ISO standard for defining generalized markup languages. An “application of SGML” means a specific tag set and rules defined using SGML. Early HTML specifications explicitly described HTML as an SGML application. Later, XHTML 1.0 recast HTML 4.01 into an XML application, but that came after the SGML lineage.

Step-by-Step Solution:

Verification / Alternative check:
Early W3C and IETF drafts, plus HTML 2.0/3.2/4.01 references, note HTML as an SGML application. XHTML 1.0 specifically calls itself an XML reformulation of HTML 4.01, confirming the timeline.

Why Other Options Are Wrong:
XHTML: A later reformulation of HTML in XML, not HTML’s original basis.
XML: A simplified subset of SGML created after HTML’s SGML roots.
None of the above: Incorrect because SGML is correct.

Common Pitfalls:
Confusing XHTML (XML-based) with classic HTML (SGML-based). Also assuming HTML5 is XML-based; HTML5 is defined via its own parsing rules, not as SGML or XML.

Final Answer:
SGML.