XML vs XHTML — Key Differences

Restored from a 2020-09 archive.

What are XML and XHTML, and how do they differ?

Both are markup languages that define web document formats. XML is a general-purpose markup language recommended by W3C for building special-purpose markup languages. XHTML combines that XML foundation with the existing features of HTML.

Traits of Classic HTML

• HTML user agents are very forgiving about errors
• Invalid tag usage, unclosed tags, or broken nesting are either ignored or quietly accepted by browsers
• Focused on how tags look rather than the meaning of the data
• Weak at structuring information, expressing relations, or validating content

HTML was originally an application of SGML. Because SGML is quite complex, most browsers didn't fully follow it — real-world HTML is a customized markup language influenced by SGML. This made HTML user-friendly but weak in extensibility and flexibility.

XML

• Created to address the limitations of HTML
• Born from the desire to go beyond the fixed HTML vocabulary
• A meta-markup language like SGML, but simplified so parsers are easier to build
• The "X" stands for extensible — you can define custom tags rather than using a fixed set
• Focuses on data transport and representation rather than visual document structure
• Content and presentation are fully separated — data structure and content are described in XML, and stylesheets drive presentation

XHTML

• A combination of HTML's familiarity with XML's rigor
• More structured syntax, stricter rules
• Improved search capability and more complex data processing
• Strict parsing means a malformed document can break completely
• When served as application/xhtml+xml or text/xml, parsed as XHTML. In IE, the type wasn't recognized and triggered download prompts, so serving IE with text/html was a common fallback