XHTML (Extensible HyperText Markup Language) is a stricter, more rigidly formatted version of HTML, requiring code to follow well-formed XML syntax rules. It was widely used in the mid-2000s, though it's since largely been superseded by the more flexible HTML5 standard.
How XHTML Differed from Standard HTML
- Required every single tag to be properly, explicitly closed
- Required all tag and attribute names to be written in lowercase
- Required all attribute values to be enclosed in quotation marks
- Enforced considerably stricter, more rigid nesting rules for elements
Why XHTML's Popularity Faded
Its strict syntax requirements made XHTML genuinely less forgiving of minor coding errors — a single unclosed tag could cause an entire page to fail outright. HTML5 later adopted many of XHTML's better practices while remaining considerably more forgiving and flexible for developers, which is a large part of why it became the clear industry standard instead.
Relevance to WordPress Today
Modern WordPress themes and the Gutenberg block editor are built around HTML5, not XHTML. XHTML is now mostly a historical footnote for anyone doing WordPress development today, though a small number of very old, long-unmaintained themes and plugins may still reference it.
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