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=== Relationship to HTML ===<!-- NOTE: this header is referenced in other articles, if you change it, make sure you do not break anything, thank you -->
HTML is the [[antecedent]] technology to XHTML. The changes from HTML to first-generation XHTML 1.0 are minor and are mainly to achieve conformance with XML. The most important change is the requirement that the document must be [[well-formed element|well-formed]] and that all [[HTML element|elements]] must be explicitly closed as required in XML.<ref>[http://www.w3.org/TR/xhtml1/#h-4.1 Documents must be well-formed - W3C ]</ref> In XML, all element and attribute names are [[case-sensitive]], so the XHTML approach has been to define all tag names to be lowercase. 

This contrasts with some earlier established traditions which began around the time of HTML 2.0, when many used uppercase tags. In XHTML, all attribute values must be enclosed by quotes; either single (<code>'</code>) or double (<code>"</code>) quotes may be used. In contrast, this was sometimes optional in SGML-based HTML, where numeric or boolean attributes can omit quotes.{{Fact|date=May 2008}} All elements must also be explicitly closed, including empty (aka [[singleton]]) elements such as <code>img</code> and <code>br</code>. This can be done by adding a closing slash to the start tag, ''e.g.'', <code>&lt;img /&gt;</code> and <code>&lt;br /&gt;</code>. Attribute minimization (e.g., <code>&lt;option selected&gt;</code>) is also prohibited, as the attribute <code>selected</code> contains no explicit value; instead this would be written as <code>&lt;option selected="selected"&gt;</code>. 

HTML elements which are optional in the content model will not appear in the [[Document Object Model|DOM]] tree unless they are explicitly specified. For example, an XHTML page ''must'' have a <code>&lt;body&gt;</code> element, and a table will not have a <code>&lt;tbody&gt;</code> element unless the author specifies one.

The XHTML 1.0 recommendation devotes a section to differences between HTML and XHTML.<ref>[http://www.w3.org/TR/xhtml1/#diffs XHTML 1.0: The Extensible HyperText Markup Language (Second Edition)<!-- Bot generated title -->]</ref>. The WHATWG wiki similarly considers differences that arise with the use of (X)HTML5.<ref>[http://wiki.whatwg.org/wiki/HTML_vs._XHTML Html Vs. Xhtml - Whatwg Wiki<!-- Bot generated title -->]</ref>.

Because XHTML and HTML are closely related technologies, sometimes they are written about and documented in parallel. In such circumstances, some authors conflate the two names by using a parenthetical notation, such as (X)HTML. This indicates that the documentation and principles can be considered to apply generally to both standards.<ref name="ExampleConflation">For examples of this, see: http://jigsaw.w3.org/css-validator/ http://www.adobe.com/devnet/dreamweaver/articles/css_best_practices_pt2.html http://www.ukoln.ac.uk/web-focus/papers/www2003/poster-1/dcmeta.doc</ref>

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