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 see wikipedia's XHTML entry.

Benefits of XHTML compliance in MediaWiki

 * elements from different XML namespaces (such as MathML and Scalable Vector Graphics) can be incorporated within articles
 * despite being a seperate specification, XHTML is viewed as the 'current' (most up-to-date) version of HTML
 * XHTML is required for many resource contrained / 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 dependant on html, and would have to be updated to use DOM
 * parsing of user supplied XHTML would have to become much stricter, badly formed markup can break an entire page

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