A simple function for getting if a document is XHTML+XML

is_xhtml.js

function getContentType() {

	var type = "text/html",
		rXHTML_XML = /application\/xhtml\+xml/i,
		element = $(document).find("meta[content][http-equiv='Content-Type']"); 
	
	if( element.length ) {
		
	 	content = element.attr("content");
		
		if( rXHTML_XML.test( content ) ) {
			type = "application/xhtml+xml";
		}	
	}
	
	return type;
}

function isXHTML() {

	var node = document.doctype,
		contentType = getContentType(),
		rDOC = /TR\/xhtml(1|11)\/DTD\/xhtml(1|11)/i,
		type = node.systemId;
		
	if( contentType === "application/xhtml+xml" || ( type && contentType === "text/html" && rDOC.test( type ) ) ) {
		return true;
	}
	
	return false;
}