Internet Explorer's support of XHTML (the successor to, and current version of, the standard document markup language HTML), is incomplete, although no claim of any support was ever made. Rather, XHTML 1.0 was intentionally designed to be usable by HTML 4.0 user agents, like Internet Explorer, if certain document authoring guidelines for backward compatibility were followed.

Furthermore, when accessing XHTML documents over a network, such support by non-XHTML-aware browsers is predicated on the documents being served with a MIME type of text/html. To the extent that these requirements are met, IE supports XHTML. XHTML has since matured, and it is now possible to author modular documents that cannot be rendered in a non-XHTML-aware browser like Internet Explorer. Furthermore, the use of the text/html MIME type is now deprecated in favor of application/xhtml+xml.

Internet Explorer does not recognize this MIME type, so instead of rendering the page, a file download prompt is presented to the user. It is possible to force IE to show application/xhtml+xml pages as either HTML or generic XML, but this workaround involves the manual editing of Windows registry. By forcing XHTML to be interpreted as HTML, it also removes the advantages of using an XML parser like well-formedness checking. Despite the advent of application/xhtml+xml, many XHTML documents on the web are still served with the text/html type in order to make XHTML documents renderable in Internet Explorer. Some consider this practice harmful as it could result in proliferation of malformatted XHTML documents.