XML

HTML · Reference cheat sheet

📋 Overview

XML (Extensible Markup Language) is a strict, generic markup language for structured data. HTML5 is an SGML-inspired living standard with its own parsing rules—far more forgiving than XML. Authors sometimes meet XML via XHTML serialization, SVG/MathML embedded in HTML, sitemaps, RSS/Atom, or app config files.

This sheet contrasts XML rules with HTML practice and covers when polyglot markup, namespaces, and well-formedness matter in web documents.

🔧 Core concepts

Well-formedness (XML)

XML documents must be well-formed:

Single root element
Properly nested tags
Attribute values quoted
Case-sensitive names
Empty elements written as <tag/> or <tag></tag>
Special characters escaped: & < > " '
Optional XML declaration: <?xml version="1.0" encoding="UTF-8"?>

HTML parsers auto-correct many errors; XML parsers fail fatally on malformed input.

HTML vs XHTML

Topic	HTML5 (text/html)	XHTML / XML
MIME	`text/html`	`application/xhtml+xml`
Void tags	`<br>` ok	`<br/>` or `<br></br>`
Omitted tags	Often allowed	Never
Namespaces	Limited magic for SVG/MathML	Explicit `xmlns`
Script/CSS	Special CDATA-like rules	Must respect XML escaping

Serving XHTML as text/html makes browsers use the HTML parser—XML rules are not enforced.

Namespaces

<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24">
  <circle cx="12" cy="12" r="10" fill="currentColor"/>
</svg>

In HTML5, inline SVG and MathML are parsed with namespace awareness. Custom elements use different rules (no XML namespaces required).

`xml:lang` and `lang`

In HTML, prefer lang. In XML vocabularies, xml:lang is standard. Polyglot documents may include both with identical values.

Embedding XML-ish content in HTML

SVG / MathML — first-class in HTML5.
Foreign data — often JSON in <script type="application/json"> instead of inline XML.
XHTML self-closing — in text/html, trailing slashes on void elements are ignored noise; on non-void elements they can create surprising DOM trees—avoid casual /> on <div/>.

💡 Examples

Minimal well-formed XML

<?xml version="1.0" encoding="UTF-8"?>
<note>
  <to>Ada</to>
  <from>Grace</from>
  <message>Ship the spec.</message>
</note>

Inline SVG in HTML

<figure>
  <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" role="img" aria-label="Green circle">
    <circle cx="50" cy="50" r="40" fill="#0f766e"/>
  </svg>
  <figcaption>Decorative brand mark</figcaption>
</figure>

Atom feed snippet

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
  <title>Docs updates</title>
  <link href="https://example.com/feed.xml" rel="self"/>
  <updated>2026-07-10T12:00:00Z</updated>
  <entry>
    <title>Forms cheat sheet</title>
    <link href="https://example.com/html/form"/>
    <id>urn:uuid:1234</id>
    <updated>2026-07-10T12:00:00Z</updated>
  </entry>
</feed>

Polyglot-minded void elements

<!-- Safe in both HTML and XHTML serializations -->
<meta charset="utf-8"/>
<link rel="stylesheet" href="/app.css"/>
<img src="/logo.svg" alt="Acme"/>
<br/>

Still serve HTML as text/html unless you intentionally need XML tooling.

⚠️ Pitfalls

Assuming HTML is XML — Unclosed <p> or <li> may “work” in HTML and break XML pipelines.
innerHTML with XML strings — HTML parser rules apply in the DOM, not XML rules.
Namespace typos — Wrong SVG xmlns yields elements in the wrong namespace (no rendering).
Raw & in XML — Must escape; HTML text content is also safer escaped.
XHTML MIME without error handling — A single error blank-screens the page in XML mode.
document.write / copy-paste XHTML — Self-closing non-void tags in HTML create empty elements and orphaned children.

Head — document prologue vs XML declaration
Language — lang / xml:lang
Meta — charset related to encoding declarations
URL — XML Base concepts
Media — SVG as image src
Script — JSON data blocks vs XML
Style — styling SVG embedded in HTML

XML

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