XHTML

MediaWiki has delivered XHTML 1.0 Transitional code since version 1.3, with small fixes and improvements towards better compliance along the way. For a full discussion of XHTML, see Wikipedia's XHTML article. However since version 1.16 MediaWiki has switched to HTML5.

Benefits of XHTML compliance in MediaWiki

 * elements from different XML namespaces (such as MathML and Scalable Vector Graphics) can be incorporated within XHTML documents.
 * despite being a separate specification, XHTML is viewed as the 'current' (most up-to-date) version of HTML.
 * XHTML is required for many resource-constrained / small form factor browsers.
 * much easier to transform (via XSLT, etc.) into other formats.

Negative aspects of delivering XHTML in MediaWiki

 * older browsers break when using XHTML mimetype.
 * many javascript extensions are dependent on HTML, and would have to be updated to use DOM.
 * parsing of user-supplied XHTML would have to become much stricter, as badly formed markup can break an entire page.

Somewhat related settings :
 * Manual:$wgDocType
 * Manual:$wgDTD