XML vs HTML — Key Differences Every Developer Should Know

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The Core Difference in Purpose

HTML (HyperText Markup Language) is a specific application for describing web pages. It has a fixed set of elements (<p>, <div>, <form>) with defined semantics, and browsers know how to render them visually.

XML (Extensible Markup Language) is a general-purpose data format. It has no predefined elements — you define your own vocabulary. XML carries data structure and meaning, not presentation. RSS feeds, SOAP messages, Android layouts, and Maven build files are all XML with application-specific vocabularies.

Side-by-Side Comparison

Tag names: HTML: fixed set defined by the spec (<p>, <table>, <video>…).
XML: you define any tag names you need (<invoice>, <product>, <quantity>).
Case sensitivity: HTML: tag and attribute names are case-insensitive — <P>, <p>, and <P> are the same element.
XML: fully case-sensitive — <Note> and <note> are distinct elements that must each have matching closing tags.
Unclosed tags: HTML: void elements like <br>, <img>, and <input> have no closing tag and the browser handles them correctly.
XML: every element must close, either with </tagname> or as a self-closing empty element <tagname/>.
Attribute values: HTML: attribute values may be unquoted if they contain no spaces, and boolean attributes like disabled need no value at all.
XML: all attribute values must be quoted (single or double) and all attributes must have explicit values.
Error handling: HTML: browsers implement a detailed error-recovery algorithm — malformed HTML is displayed as best the browser can guess.
XML: parsers stop immediately on any well-formedness error and report a fatal error. There is no recovery.
Multiple root elements: HTML: a page is implicitly rooted at <html>, and parsers handle fragments gracefully.
XML: exactly one root element is required. A document with two top-level elements is invalid.
Whitespace handling: HTML: collapsing of whitespace is defined per-element by CSS display rules.
XML: whitespace is significant and preserved by default unless the parser or application explicitly strips it.

The XHTML Middle Ground

XHTML is HTML reformulated as a valid XML application. An XHTML document must obey XML's strict rules — all tags closed, all attributes quoted, exactly one root element — while using HTML's element vocabulary (<p>, <div>, etc.).

XHTML was popular in the early 2000s as a stepping stone toward modular web formats. It fell out of favor with HTML5's arrival. You may still encounter XHTML in legacy codebases or feed validators — serve it as application/xhtml+xml, not text/html, to get strict XML parsing in browsers.

When to Use XML vs HTML

Use HTML for anything displayed in a browser as a web page — content, forms, dashboards, and applications rendered visually by browser engines.
Use XML for data interchange between systems (APIs, feeds, configuration), document formats with semantic structure (DOCX, SVG, EPUB), and any format that needs a custom schema.
Do not try to parse HTML with an XML parser. Most real-world HTML is not well-formed XML — use an HTML-specific parser (like DOMParser with text/html) for HTML, and an XML parser for XML.

SVG: Where XML and the Browser Meet

SVG (Scalable Vector Graphics) is the primary XML format that lives inside web pages. Inline SVG embedded in HTML must follow XML rules — self-closing tags, quoted attributes, no overlapping elements. This is one of the few cases where browser developers deal with strict XML parsing alongside lenient HTML parsing in the same document.

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