manifest.json

{
  "name": "XKCD Text",
  "version": "0.1",
  "permissions": [
   "http://*/*"    
  ],
  "content_scripts": [
    {
      "matches": ["http://*/*"],
      "js": ["xkcdfy.js"],
      "run_at": "document_end"
    }
  ],
  "manifest_version": 2
}

xkcdfy.js

String.prototype.replaceWithCase=function(subStr, newStr) {
 return this.replace( new RegExp(subStr, 'ig'), function(found) {
  return /[A-Z]/.test(found.charAt(0))?( newStr.charAt(0).toUpperCase() + newStr.substring(1) ) : newStr.toLowerCase();
 });  
}

var replacements = [["witnesses", "these dudes I know"],
                    ["allegedly", "kinda probably"],
                    ["new study", "tumbler 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 reached for comment", "is guilty and everyone knows it"]];

function replaceAll (text) {
  var result = text;
  for (r in replacements) {
    var pair = replacements[r];
    result = result.replaceWithCase(pair[0], pair[1]);
  }
  return result;
}


document.body.innerHTML = replaceAll(document.body.innerHTML);