# File lib/premailer/premailer.rb, line 397
  def is_xhtml?
    intro = @doc.to_html.strip.split("\n")[0..2].join(' ')
    is_xhtml = !!(intro =~ /w3c\/\/[\s]*dtd[\s]+xhtml/i)
    $stderr.puts "Is XHTML? #{is_xhtml.inspect}\nChecked:\n#{intro}" if @options[:debug]
    is_xhtml
  end