main part of the injection script

inject.js

// Client script to find all the styles of a page and compute the position of the matching elements

(function(window){
  var document = window.document;
  
  //reinsert any cross domain restricted sheets
  var reinsert = []
  var sheets = document.styleSheets;
  for (var i=0; i < sheets.length; i++) {
    if(sheets[i].cssRules == null){
      reinsert.push(sheets[i]);
    }
  };
  for (var i=0; i < reinsert.length; i++) {
    var sheet = reinsert[i];
    
    var callback = function(){
      try {
        var newsheet = document.createElement('style');
            newsheet.setAttribute('type','text/css');
            newsheet.innerHTML = this.responseText;
            
        // insert before so that any relative images that will now fail
        // will be overridden 
        sheet.ownerNode.parentNode.insertBefore(newsheet, sheet.ownerNode);
      } catch(e) {
        // alert(e); // "Error: SYNTAX_ERR: DOM Exception 12"
      }
    };


    var oXHR = new XMLHttpRequest();
    oXHR.onreadystatechange = function() {
      if (oXHR.readyState === 4) { callback.apply(oXHR); }
    };

    console.log(window._wtcss_prox + sheet.href);

    oXHR.open("GET", window._wtcss_prox + sheet.href, false);// synchronously
    oXHR.send(null);

  };
  
    
  // where we will put all the styles
  window._wtcss_styles = [];
  
  (function process(sheets,aggregated){
    
    for (var i=0; i < sheets.length; i++) {
      if(sheets[i].cssRules == null){
        // TODO - re-insert unavailable (cross domain) styles
        continue;
      }
      
      var rules = sheets[i].cssRules;
      
      for(var j = 0; j < rules.length; j++){
        var rule = rules[j];
        
        if(rule.selectorText){
          var rr = [rule.cssText];
          try {
            var elements = document.querySelectorAll(rule.selectorText);
            for(var k = 0; k < elements.length; k++){
             var offset = offsets(elements[k]);
             rr.push(offset);
            }
            aggregated.push(rr);
          } catch(e) {}
        }
        
        // recursively add @import styles
        if(rule.styleSheet){
          process([rule.styleSheet], aggregated);
        }
      }
    }
    
  })(document.styleSheets,window._wtcss_styles);
  
  // the full offsets of a given element
  // returns [top, left]
  function offsets(elem){
    if(!elem) return [0,0];
    var vals = offsets(elem.offsetParent);
    return [vals[0] + elem.offsetTop, vals[1] + elem.offsetLeft];
  };
  
  
})(window)