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The similarities between HTML 4.01 and XHTML 1.0 led many web sites and content management systems to adopt the initial W3C XHTML 1.0 Recommendation. To aid authors in the transition, the W3C provided guidance on how to publish XHTML 1.0 documents in an HTML-compatible manner, and serve them to browsers that were not designed for XHTML.
Such "HTML-compatible" content is sent using the HTML media type (text/html) rather than the official Internet media type for XHTML (application/xhtml+xml). When measuring the adoption of XHTML to that of regular HTML, therefore, it is important to distinguish whether it is media type usage or actual document contents that is being compared.
Most web browsers have mature support for all of the possible XHTML media types. The notable exception is Internet Explorer versions 8 and earlier by Microsoft; rather than rendering application/xhtml+xml content, a dialog box invites the user to save the content to disk instead. Both Internet Explorer 7 (released in 2006) and Internet Explorer 8 (released in March 2009) exhibit this behavior. Microsoft developer Chris Wilson explained in 2005 that IE7’s priorities were improved security and CSS support, and that proper XHTML support would be difficult to graft onto IE’s compatibility-oriented HTML parser; however, Microsoft has added support for true XHTML in developer previews of IE9 that has carried over to the released version of IE9.
As long as support is not widespread, most web developers avoid using XHTML that isn’t HTML-compatible, so advantages of XML such as namespaces, faster parsing and smaller-footprint browsers do not benefit the user.