gistfile1.rb

require 'httparty'
require 'rack'

class Rack::Validate
  
  attr_reader :app, :options
  
  def initialize(app, options={})
    @app     = app
    @options = options
  end
  
  def call(env)
    code, headers, body = app.call(env)
    headers['X-W3-Valid'] = invalid_markup?(body) ? 'false' : 'true'
  ensure
    return code, headers, body
  end
  
private ######################################################################

  def invalid_markup?(html)
    query  = { :fragment => html, :output => 'soap12' }
    result = HTTParty.post('http://validator.w3.org/check', :body => query)
    result.to_s.index('<m:validity>true</m:validity>').nil?
  end

end