Simple function to determine if a given Document object in JavaScript represents an XHTML document.

isXhtmlDoc.js

//
// References important to this particular implementation:
//  - http://ejohn.org/blog/nodename-case-sensitivity/
//  - http://help.dottoro.com/ljdgsrle.php
//  - http://www.w3schools.com/html/html_xhtml.asp
//  - http://www.w3schools.com/tags/tag_doctype.asp
//
// Other related references if additional parameters are ever needed:
//  - http://reference.sitepoint.com/javascript/Document/doctype
//  - http://reference.sitepoint.com/javascript/DocumentType
//  - http://help.dottoro.com/ljlsvbgj.php
//  - http://nimbupani.com/the-truth-about-doctypes.html
//

// Implementation explanation:
//  - If a document is being rendered in HTML mode, then all `.nodeName` properties
//    return their values in all uppercase (e.g. `HTML`).
//  - If a document is being rendered in XML/XHTML mode, then all `.nodeName` properties
//    return their values in their original case. However, valid XHTML also requires all
//    element names to be specified as lowercase, so the original case MUST always be
//    lowercase (e.g. `html`).
function isXhtmlDoc( doc ) {
  var _doc = doc || ( typeof document !== 'undefined' && document );
  return _doc && _doc.documentElement && _doc.documentElement.nodeName === 'html';
}