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The Extensible HyperText Markup Language (XHTML) is form of web page markup language which is similar to HTML but adheres to stricter syntax rules, being based on XML. The specifications for XHTML are maintained by the W3C, who describe XHTML 1.0 as "a reformulation of HTML 4 as an XML 1.0 application". Well-formed XHTML 1.0 can be parsed accurately by Javascript, allowing programming operations on the HTML Document Object Model (DOM), and as such, it represented a step forward as compared with prior versions of HTML, which could not be parsed as reliably in code.

The semantics of the XHTML 1.0 elements and their attributes are not defined separately from those for HTML, being the same as those in the W3C Recommendation for HTML 4.This makes XHTML compatible with existing HTML user agents, if certain guidelines are followed.

There are two XHTML versions, 1.0 and 1.1. The latter, version 1.1, was little used. As of 2011, there seems to be a trend to replace XHTML 1.0 in favor of HTML5, though many unresolved issues remain (such as HTML5 incompatibility with RDF) and thus XHTML 1.0 is still in wide use.