diff --git a/CHANGELOG b/CHANGELOG
index 8a1635e..a23a2b5 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,11 @@
 *SVN*
 
+* Make document.getElementsByClassName match the WHATWG Web Applications 1.0 specification which was adopted in Firefox 3
+(http://www.whatwg.org/specs/web-apps/current-work/#getelementsbyclassname). It now supports multiple class names given as an array or a space-separated list in a string. The method will only return the nodes that match all the class names. In browsers that implement the method natively it will not be overwritten.  Closes #8401.  [Mislav Marohnić]
+  Examples:
+    document.getElementsByClassName('foo bar')
+    document.getElementsByClassName(['foo', 'bar'])
+
 * Fix a Safari rendering issue when floating elements could temporarily disappear when opacity was set to 1.  Closes #7063.  References #3044, #3813, #6706.  [Thomas Fuchs, davidjrice]
 
 * Prevent a crash in Safari when calling String#evalJSON(true) on very large strings. Add String#isJSON.  Closes #7834.  [Tobie Langel]
diff --git a/src/dom.js b/src/dom.js
index 7a608b8..f14fb80 100644
--- a/src/dom.js
+++ b/src/dom.js
@@ -18,22 +18,7 @@ if (Prototype.BrowserFeatures.XPath) {
       results.push(query.snapshotItem(i));
     return results;
   };
-  
-  document.getElementsByClassName = function(className, parentElement) {
-    var q = ".//*[contains(concat(' ', @class, ' '), ' " + className + " ')]";
-    return document._getElementsByXPath(q, parentElement);
-  }
-  
-} else document.getElementsByClassName = function(className, parentElement) {
-  var children = ($(parentElement) || document.body).getElementsByTagName('*');
-  var elements = [], child;
-  for (var i = 0, length = children.length; i < length; i++) {
-    child = children[i];
-    if (Element.hasClassName(child, className))
-      elements.push(Element.extend(child));
-  }
-  return elements;
-};
+}
 
 /*--------------------------------------------------------------------------*/
 
@@ -243,10 +228,6 @@ Element.Methods = {
     return Selector.findChildElements(element, args);
   },
   
-  getElementsByClassName: function(element, className) {
-    return document.getElementsByClassName(className, element);
-  },
-  
   readAttribute: function(element, name) {
     element = $(element);
     if (Prototype.Browser.IE) {
@@ -614,6 +595,42 @@ Element.Methods = {
   }
 };
 
+if (!document.getElementsByClassName) document.getElementsByClassName = function(instanceMethods){
+  function isArray(className) {
+    return className.constructor == Array || (/\s/.test(className) && !className.toString().blank());
+  }
+  function classNamesArray(classNames) {
+    return classNames.constructor == Array ? classNames : $w(classNames.toString());
+  }
+  function iter(name) {
+    return name.toString().blank() ? null : "[contains(concat(' ', @class, ' '), ' " + name + " ')]";
+  }
+
+  instanceMethods.getElementsByClassName = Prototype.BrowserFeatures.XPath ?
+  function(element, className) {
+    var cond = isArray(className) ? classNamesArray(className).map(iter).join('') : iter(className);
+    return cond ? document._getElementsByXPath('.//*' + cond, element) : [];
+  } : function(element, className) {
+    var elements = [], classNames = (isArray(className) ? classNamesArray(className) : null);
+    if (classNames ? !classNames.length : className.toString().blank()) return elements;
+    var nodes = $(element).getElementsByTagName('*');
+    className = ' ' + (classNames ? classNames.join(' ') : className) + ' ';
+
+    for (var i = 0, child, cn; child = nodes[i]; i++) {
+      if (child.className && (cn = ' ' + child.className + ' ') && (cn.include(className) ||
+          (classNames && classNames.all(function(name) {
+            return !name.toString().blank() && cn.include(' ' + name + ' ');
+          }))))
+        elements.push(Element.extend(child));
+    }
+    return elements;
+  };
+
+  return function(className, parentElement) {
+    return $(parentElement || document.body).getElementsByClassName(className);
+  };
+}(Element.Methods);
+
 Object.extend(Element.Methods, {
   childElements: Element.Methods.immediateDescendants
 });
diff --git a/test/lib/unittest.js b/test/lib/unittest.js
index 75b7dc4..1be1fc6 100644
--- a/test/lib/unittest.js
+++ b/test/lib/unittest.js
@@ -484,7 +484,7 @@ Test.Unit.Assertions.prototype = {
   assertElementsMatch: function() {
     var expressions = $A(arguments), elements = $A(expressions.shift());
     if (elements.length != expressions.length) {
-      this.fail('assertElementsMatch: size mismatch: ' + elements.length + ' elements, ' + expressions.length + ' expressions');
+      this.fail('assertElementsMatch: size mismatch: ' + elements.length + ' elements, ' + expressions.length + ' expressions (' + expressions.inspect() + ')');
       return false;
     }
     elements.zip(expressions).all(function(pair, index) {
diff --git a/test/unit/dom.html b/test/unit/dom.html
index 0f3eac4..06baeb0 100644
--- a/test/unit/dom.html
+++ b/test/unit/dom.html
@@ -207,8 +207,10 @@
   <ul class="A B" id="class_names_ul">
     <li class="C"></li>
     <li class="A C"></li>
+    <li class="1"></li>
   </ul>
   <div class="B C D"></div>
+  <div id="unextended"></div>
 </div>
 
 <div id="style_test_1" style="display:none;"></div>
@@ -337,7 +339,9 @@
 </div>
 <div id="notInlineAbsoluted"></div>
 <div id="inlineAbsoluted" style="position: absolute"></div>
- 
+
+<div id="unextended"></div>
+
 <!-- Tests follow -->
 <script type="text/javascript" language="javascript" charset="utf-8">
 // <![CDATA[
@@ -381,10 +385,31 @@
     }},
     
     testGetElementsByClassName: function() {with(this) {
+      var div = $('class_names'), list = $('class_names_ul');
+
       assertElementsMatch(document.getElementsByClassName('A'), 'p.A', 'ul#class_names_ul.A', 'li.A.C');
-      assertElementsMatch(document.getElementsByClassName('A', 'class_names_ul'), 'li.A.C');
-      assertElementsMatch(document.getElementsByClassName('B', 'class_names'), 'ul#class_names_ul.A.B', 'div.B.C.D');
-      assertElementsMatch(document.getElementsByClassName('B', 'class_names_ul'));
+      
+      if (Prototype.Browser.IE)
+        assertUndefined(document.getElementById('unextended').show);
+      
+      assertElementsMatch(list.getElementsByClassName('A'), 'li.A.C');
+      assertElementsMatch(list.getElementsByClassName('C A'), 'li.A.C');
+      assertElementsMatch(div.getElementsByClassName('B'), 'ul#class_names_ul.A.B', 'div.B.C.D');
+      assertElementsMatch(div.getElementsByClassName('D C B'), 'div.B.C.D');
+      assertElementsMatch(div.getElementsByClassName($w('D C B')), 'div.B.C.D');
+      assertElementsMatch(list.getElementsByClassName('B'));
+      assertElementsMatch(list.getElementsByClassName('1'), 'li.1');
+      assertElementsMatch(list.getElementsByClassName([1]), 'li.1');
+      assertElementsMatch(list.getElementsByClassName(['1 junk']));
+      assertElementsMatch(list.getElementsByClassName(''));
+      assertElementsMatch(list.getElementsByClassName(' '));
+      assertElementsMatch(list.getElementsByClassName(['']));
+      assertElementsMatch(list.getElementsByClassName([' ', '']));
+      assertElementsMatch(list.getElementsByClassName({}));
+      assertEqual(Array, [].constructor)
+
+      // those lookups shouldn't have extended all nodes in document
+      if (Prototype.Browser.IE) assertUndefined(document.getElementById('unextended')['show']);
     }},
 
     testElementInsertWithHTML: function() {with(this) {