Differences between HTML and XHTML

Comparison Article

HTML (HyperText Markup Language) and XHTML (Extensible HyperText Markup Language) are markup languages used to structure and present content on the World Wide Web. While they share similar element names and purposes, they differ in their underlying syntax and technical constraints. HTML is an application of Standard Generalized Markup Language (SGML), whereas XHTML is a reformulation of HTML as an application of XML.

Development and history

The World Wide Web Consortium (W3C) published the first version of the XHTML 1.0 specification in January 2000. It intended to replace HTML 4.01 by providing a more rigorous structure that could be parsed by standard XML tools. During the mid-2000s, the W3C focused on XHTML 2.0, which was not backward compatible with previous web standards. This led to a schism in web development, resulting in the formation of the WHATWG, which developed the "HTML Living Standard" that eventually became HTML5.

Technical differences

The primary distinction between the two languages lies in the strictness of their syntax. HTML allows for certain shortcuts, such as omitting closing tags for specific elements (e.g.,

or

) or using "tag soup" formatting that browsers attempt to correct during rendering. In contrast, XHTML requires documents to be "well-formed" according to XML rules. This means every element must be explicitly closed, and empty elements must use a trailing slash (e.g.,
). XHTML is also case-sensitive, requiring all element and attribute names to be in lowercase, whereas HTML is case-insensitive.

Comparison table

Feature	HTML	XHTML
Document Type	Based on SGML	Based on XML
Case Sensitivity	Case-insensitive	Case-sensitive (lowercase required)
Closing Tags	Optional for some elements	Required for all elements
Attribute Quoting	Optional if no spaces are present	Mandatory for all values
Empty Elements	Written as	Written as
Error Handling	Forgiving; browsers repair markup	Strict; parsing fails on errors
Attribute Minimization	Allowed (e.g., checked)	Prohibited (e.g., checked="checked")
MIME Type	text/html	application/xhtml+xml or text/xml

Venn diagram comparing Differences between HTML and XHTML

Parsing and MIME types

Most browsers determine how to parse a document based on the media type (MIME type) sent by the web server. If a document is served as text/html, the browser uses an HTML parser, even if the code follows XHTML syntax. True XHTML requires the application/xhtml+xml MIME type. When a browser encounters a syntax error in a true XHTML document, XML specifications dictate that the browser should stop processing the page and display an error message, rather than attempting to guess the author's intent as an HTML parser would.

Convergence in HTML5

Current web development practices under the HTML5 standard have largely merged these two approaches. HTML5 defines both an HTML syntax and an XML-compatible syntax (often called XHTML5). While the strict requirements of XHTML are less common in modern front-end development, the principles of well-formed code are still used in various data exchange formats and content management systems.

References