Yes, the W3C recommends using XHTML which is 'a reformulation of HTML 4 in XML 1.0'. This specification defines HTML as an XML application, and provides three DTDs corresponding to the ones defined by HTML 4.* (Strict, Transitional, and Frameset). 

Yes, if the document type you use provides for math, and your users' browsers are capable of rendering it. The mathematics-using community has developed the MathML Recommendation at the W3C, which is a native XML application suitable for embedding in other DTDs and Schemas.

XML is a project of the World Wide Web Consortium (W3C), and the development of the specification is supervised by an XML Working Group. XML is a public format: it is not a proprietary development of any company, although the membership of the WG and the SIG represented companies as well as research and academic institutions