see: http://xkcd.com/1288/

substitutions.rb

#!/usr/bin/ruby

# dependencies:
# gem install sinatra
# gem install mechanize

#
# to run:
#   from command line type: ruby substitutions.rb &
#   and in your browser, go to: localhost:4567/http://some.random.site.com/
#

require 'uri'
require 'sinatra'
require 'mechanize'

$debug = !!ARGV.delete('debug')
def debug(message)
  $stderr.puts(message) if $debug
end

SUBSTITUTIONS = {
  'witnesses' => 'these dudes I know',
  'allegedly' => 'kinda probably',
  'new study' => 'tumblr post',
  'rebuild'   => 'avenge',
  'space'     => 'spaaace',
  'google glass' => 'virtual boy',
  'smartphone' => 'pokedex',
  'electric' => 'atomic',
  'senator' => 'elf-lord',
  'car' => 'cat',
  'election' => 'eating contest',
  'congressional leaders' => 'river spirits',
  'homeland security' => 'homestar runner',
  'could not be reach for comment' => 'is guilty and everyone knows it'
}

agent = Mechanize.new { |agent|
  agent.user_agent = '1288-Bot/1.0 (+https://gist.github.com/rabbitt/7457688)'
}

get '/*' do
  url = params[:splat].first.gsub(%r{^(https?):[^a-z0-9-]*}, "\\1://")
  url = URI.parse(url.include?('http') ? url : "http://#{url}")

  root = agent.get(url.to_s).root

  root.at_xpath('//head').tap { |head|
    base = Nokogiri::XML::Node.new('base', head)
    base['href'] = url.dup.tap { |u| u.query = ''; u.path = '/' }.to_s
    head.add_child(base)

    styles = Nokogiri::XML::Node.new('style', head)
    styles.content = <<-'EOS'.gsub('^\s{6}', '')
      .xkcd-replacement { background-color: pink; }
    EOS
    head.add_child(styles)

    jquery = Nokogiri::XML::Node.new('script', head)
    jquery['src'] = 'http://code.jquery.com/jquery-2.0.3.min.js'
    head.add_child(jquery)
  }

  root.at_xpath('//body').tap { |body|
    js = Nokogiri::XML::Node.new('script', body)
    js.content = <<-'EOF'.gsub(/^.{6}/, '')
      $('.xkcd-replacement').hover(function() { $(this).siblings('.xkcd-original').show(); $(this).hide(); });
      $('.xkcd-original').hover(function() {}, function() { $(this).siblings('.xkcd-replacement').show(); $(this).hide(); });
    EOF
    body.add_child(js)
  }

  root.tap { |html|
    page = html.to_html

    SUBSTITUTIONS.each {|search, replace|
      # find all uniq matches (ie: Google Glass, GOOGLE GLASS, google glass, etc)
      page.scan(/#{search}/i).uniq.each do |match|
        replacement = case match
          # if it looks like a title style capitalized match, then capitalize all replacement words
          when /^[A-Z][a-z]+/ then replace.split(/\s+/).map(&:capitalize).join(' ')
          # if it looks like it's fully uppercase, then fully uppercase the replacement
          when /^[A-Z]+\b/ then replace.upcase
          # otherwise just replace it as is
          else replace
        end

        debug "  --> Searching for elements containing #{match.inspect}"
        html.xpath(%Q{//*[contains(text(), "#{match}")]}).inject([]) { |list,element|
          debug "  --> Found Element #{element.name} with match #{match} - path: #{element.path}"
          next list if (element.path.include?('/head/') || %w(style script iframe object noscript).include?(element.name))
          list.tap { |l| l << element } 
        }.each do |element|

          repl_node = Nokogiri::XML::Node.new('span', element).tap {|node|
            node['class'] = 'xkcd-replacement'
            node.content  = replacement
          }

          orig_node = Nokogiri::XML::Node.new('span', element).tap {|node|
            node['class'] = 'xkcd-original'
            node['style'] = 'display:none;'
            node.content  = match
          }

          children = element.content.split(/(#{Regexp.escape(match)})/).reject(&:empty?)
          children = children.collect {|part|
            part == match ? [repl_node.dup, orig_node.dup] : Nokogiri::XML::Text.new(part, element.parent)
          }.flatten

          element.children = Nokogiri::XML::NodeSet.new(element.document, children.flatten)
          debug "    --> Found #{match.inspect} and repalceed with #{replacement.inspect}\n      --> Modified Element (#{element.name}): #{element.to_html.inspect}"
        end
      end
    }
  }.to_html
end

Dependencies:

gem install mechanize
gem install sinatra

Run from command line:
ruby substitutions.rb [debug]

Then open your browser to:

http://localhost:4567/<url>

Examples:

http://localhost:4567/http://www.foxnews.com/politics/2013/11/11/white-texas-republican-wins-election-with-campaign-that-implied-black/
http://localhost:4567/http://www.foxnews.com/politics/2013/11/13/senator-says-us-ending-buys-russian-helicopters/