A more useful DocumentType.prototype.toString implementation than present-day browsers provide.

document.doctype.toString.coffee

# browser portability notes on this here: http://help.dottoro.com/ljlsvbgj.php
window.DocumentType::toString = ->
  pub = " #{JSON.stringify pub}"  if pub = @publicId or ''
  sys = " #{JSON.stringify sys}"  if sys = @systemId or ''
  availability = ''
  availability = ' SYSTEM'  if sys
  availability = ' PUBLIC'  if pub
  declarations = " [#{declarations}]"  if declarations = @internalSubset or ''
  "<!DOCTYPE #{@name}#{availability}#{pub}#{sys}#{declarations}>"

document.doctype.toString.js

// browser portability notes on this here: http://help.dottoro.com/ljlsvbgj.php
window.DocumentType.prototype.toString = function() {
  var pub = this.publicId || ''
    , sys = this.systemId || ''
    , availability = pub ? ' PUBLIC' : sys ? ' SYSTEM' : ''
    , declarations = this.internalSubset || ''
    ;
  if (pub) pub = ' ' + JSON.stringify(pub);
  if (sys) sys = ' ' + JSON.stringify(sys);
  if (declarations) declarations = ' [' + declarations + ']';
  return '<!DOCTYPE ' + this.name + availability + pub + sys + declarations + '>';
};