function createCustomRuby( frag ) {
  var $ruby = frag.firstChild
  var hruby = $.create( 'h-ruby' )

  hruby.innerHTML = $ruby.innerHTML
  $.setAttr( hruby, $ruby.attributes )
  hruby.normalize()
  return hruby
}
Create and return a new `<h-ru>` element
according to the given contents