reverse.user.js

// ==UserScript==
// @name        Reverse Web
// @namespace   http://www.example.com
// @grant
// ==/UserScript==

function reverseWordButKeepCase(str) {
   var buffer = "";
   for (var i = 0, j = str.length - 1; i < str.length; i++, j--) {
     if (str.charAt(i) == str.charAt(i).toUpperCase()) {
       buffer += str.charAt(j).toUpperCase();
     }
     else {
       buffer += str.charAt(j).toLowerCase();
     }
   }
   return buffer;
}

function reverseText(str) {
    return str.replace(/\w+/g,reverseWordButKeepCase)
}

var path = "//text()[not(ancestor::script or ancestor::style)]"
var arr = document.evaluate(path, document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);

for (var i = 0; i < arr.snapshotLength; i++) {
  var each = arr.snapshotItem(i)
  each.textContent = reverseText(each.textContent)
}