names=name
visibility=public
kind=defined

--- name -> String
ルート要素名を返します。

  document = REXML::Document.new(<<EOS)
  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 
           "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
  EOS
  doctype = document.doctype
  p doctype.name # => "html"

