Markup Languages: Differences, Uses, and Conversions Between XML, Markdown, and HTML

Markup languages are essential for structuring, organizing, and rendering information in digital environments. The most commonly used are XML, Markdown, and HTML, each with distinct goals and characteristics. This article compares their main uses, conceptual and technical differences, and the common issues faced when converting between them.

1. What is XML?

XML (eXtensible Markup Language) is designed to store, transport, and structure data in a hierarchical and extensible format. Its syntax is strict, and it focuses on structure rather than presentation.

Common uses:

  • Data exchange between heterogeneous systems.
  • Structured representation of information (legal texts, books, configurations).
  • Foundation for derived languages like TEI, MathML, SVG, RSS.
<book>
  <title>Don Quixote</title>
  <author>Miguel de Cervantes</author>
  <year>1605</year>
</book>

XML and the TEI Standard

One of the most sophisticated uses of XML is in the TEI (Text Encoding Initiative) standard. TEI defines a comprehensive tag set to represent literary, historical, or philosophical texts with rich metadata and deep hierarchical structure.

Common applications of TEI:

  • Critical editions of classical works.
  • Digitization of manuscripts and historical archives.
  • Digital humanities and computational philology projects.

TEI Example:

<TEI xmlns="http://www.tei-c.org/ns/1.0">
  <teiHeader>
    <fileDesc>
      <titleStmt>
        <title>Don Quixote</title>
        <author>Miguel de Cervantes</author>
      </titleStmt>
      <publicationStmt>
        <publisher>Digital Library Project</publisher>
      </publicationStmt>
    </fileDesc>
  </teiHeader>
  <text>
    <body>
      <p>In a village of La Mancha...</p>
    </body>
  </text>
</TEI>

2. What is Markdown?

Markdown is a lightweight markup language designed to be easy to write and read in plain text. It enables simple conversion to HTML without requiring complex tags.

Common uses:

  • Rapid content creation for blogs, forums, technical documentation.
  • Static site generation (Hugo, Jekyll).
  • Academic note-taking and collaborative editing.
# Don Quixote

**Author:** Miguel de Cervantes  
*Year:* 1605

CommonMark and Pandoc Extensions

Markdown is not a single standard: multiple variants exist. CommonMark is a formal specification that aims to unify them. Tools like Pandoc extend Markdown with advanced features such as footnotes, citations, LaTeX equations, and export to multiple formats.

Advantages of using Pandoc:

  • Convert to PDF, HTML, DOCX, LaTeX, EPUB, XML, etc.
  • Supports .bib bibliographies, CSL citations, footnotes.
  • Lua/JSON filters for document transformation.

Extended syntax example:

[^1]: Footnote with bibliographic reference.

> Blockquote with author:  
> — Aristotle, *Nicomachean Ethics*

3. What is HTML?

HTML (HyperText Markup Language) is the standard for web pages. It defines the structure and presentation of content for browsers, including text, images, links, forms, and multimedia.

Common uses:

  • Web development.
  • Content rendering in browsers.
  • Frontend structure (enhanced by CSS and JavaScript).
<h1>Don Quixote</h1>
<p><strong>Author:</strong> Miguel de Cervantes</p>
<p><em>Year:</em> 1605</p>

HTML5 and the Shift from XHTML

HTML5 is the modern version of the language, focused on semantics, accessibility, and multimedia support. Unlike XHTML, which follows XML’s strict rules, HTML5 is forgiving and browser-friendly.

Key differences between HTML5 and XHTML:

  • HTML5 allows unclosed tags (<br>, <img>) and unquoted attributes.
  • XHTML requires well-formed XML.
  • HTML5 introduces semantic tags like <section>, <article>, <nav>, <aside>.

HTML5 Example:

<article>
  <h2>Blog Entry</h2>
  <p>Published on <time datetime="2025-06-02">June 2025</time></p>
</article>

4. Key Differences

FeatureXMLMarkdownHTML
Main purposeStructured data storageLightweight text writingWeb content presentation
SyntaxStrict, hierarchicalInformal, minimalisticStructured, visual
Nesting capabilityFully nestedLimitedFully nested
Formal standardW3C, strict complianceInformal, extensibleW3C-compliant
Human readabilityMediumHighLow to medium
ExtensibilityHigh (custom tags allowed)LimitedModerate

5. Conversion Challenges

Markdown → HTML

✅ Well-supported with many tools
⚠️ Loses complex structures, inconsistent extensions

HTML → Markdown

✅ Possible with tools like Pandoc
⚠️ Attributes, classes, and some structures may be lost

XML → HTML

✅ Feasible with XSLT or custom scripts
⚠️ Requires semantic mapping from XML to HTML

HTML → XML

✅ Only if well-formed (XHTML)
⚠️ Most HTML needs cleanup to be valid XML

Markdown → XML

✅ Possible via adapters
⚠️ Markdown lacks explicit hierarchy and metadata

XML → Markdown

⚠️ Difficult

  • Rich XML structures don’t translate to flat Markdown
  • Needs extraction rules and simplification

6. Conclusion

While XML, Markdown, and HTML are all markup languages, their design and goals differ:

  • XML prioritizes structure and validation.
  • Markdown emphasizes simplicity and readability.
  • HTML focuses on web presentation.

Converting between them often involves trade-offs, structure loss, or intermediate tools. In digital humanities or structured publishing workflows, it’s essential to choose the right format based on whether the goal is rapid writing (Markdown), web display (HTML), or long-term preservation (XML).