> It’s not failed: it’s the only language used in the ePub standard.
"XHTML" can refer to both a failed series of standards intended to replace HTML (XHTML 1.0, XHMTL 1.1, and XHMTL 2.0) and the XML syntax of HTML that is part of the HTML living standard, which itself resulted from the failure of the XHMTL series of standards. EPUB 3's "XHTML" is the latter, not the former.
"XHTML" can refer to both a failed series of standards intended to replace HTML (XHTML 1.0, XHMTL 1.1, and XHMTL 2.0) and the XML syntax of HTML that is part of the HTML living standard, which itself resulted from the failure of the XHMTL series of standards. EPUB 3's "XHTML" is the latter, not the former.