diff --git a/library/HTMLPurifier/AttrDef/Class.php b/library/HTMLPurifier/AttrDef/Class.php
new file mode 100644
index 00000000..c6cbe916
--- /dev/null
+++ b/library/HTMLPurifier/AttrDef/Class.php
@@ -0,0 +1,43 @@
+<?php
+
+require_once 'HTMLPurifier/AttrDef.php';
+require_once 'HTMLPurifier/Config.php';
+
+class HTMLPurifier_AttrDef_Class extends HTMLPurifier_AttrDef
+{
+    
+    function validate($raw_string, $config = null) {
+        
+        if (!$config) $config = HTMLPurifier_Config::createDefault();
+        
+        $string = trim($raw_string);
+        
+        // early abort: '' and '0' (strings that convert to false) are invalid
+        if (!$string) return false;
+        
+        // OPTIMIZABLE!
+        // do the preg_match, capture all subpatterns for reformulation
+        
+        // we don't support U+00A1 and up codepoints or
+        // escaping because I don't know how to do that with regexps
+        // and plus it would complicate optimization efforts (you never
+        // see that anyway).
+        $matches = array();
+        $pattern = '/(?:\s|\A)'.
+                   '((?:-?[A-Za-z_]|--)[A-Za-z_\-0-9]*)'.
+                   '(?:\s|\z)/';
+        preg_match_all($pattern, $string, $matches);
+        
+        $new_string = '';
+        foreach ($matches[1] as $class_names) {
+            $new_string .= $class_names . ' ';
+        }
+        $new_string = rtrim($new_string);
+        
+        return ($new_string == $raw_string) ? true : $new_string ? $new_string : false;
+        
+    }
+    
+}
+
+?>
\ No newline at end of file
diff --git a/library/HTMLPurifier/AttrDef/ID.php b/library/HTMLPurifier/AttrDef/ID.php
index a7815027..2c90e279 100644
--- a/library/HTMLPurifier/AttrDef/ID.php
+++ b/library/HTMLPurifier/AttrDef/ID.php
@@ -2,6 +2,12 @@
 
 require_once 'HTMLPurifier/AttrDef.php';
 require_once 'HTMLPurifier/IDAccumulator.php';
+        
+// NOTE QUIRKY BEHAVIOR: even though this is the id processor, it
+// will ignore HTMLPurifier_Config::$attr_id_blacklist: it will only
+// go according to the ID accumulator. Since the accumulator is
+// automatically generated, it will have already absorbed the
+// blacklist. If you're hacking around, make sure you use load()!
 
 class HTMLPurifier_AttrDef_ID extends HTMLPurifier_AttrDef
 {
diff --git a/library/HTMLPurifier/Definition.php b/library/HTMLPurifier/Definition.php
index 99f67d1e..255ff4ff 100644
--- a/library/HTMLPurifier/Definition.php
+++ b/library/HTMLPurifier/Definition.php
@@ -237,6 +237,7 @@ class HTMLPurifier_Definition
         $this->info_global_attr = array(
             // core attrs
             'id' => new HTMLPurifier_AttrDef_ID(),
+            'class' => new HTMLPurifier_AttrDef_Class(),
             // i18n
             'dir' => new HTMLPurifier_AttrDef_Enum(array('ltr','rtl'), false),
             );
diff --git a/tests/HTMLPurifier/AttrDef/ClassTest.php b/tests/HTMLPurifier/AttrDef/ClassTest.php
new file mode 100644
index 00000000..6de74351
--- /dev/null
+++ b/tests/HTMLPurifier/AttrDef/ClassTest.php
@@ -0,0 +1,32 @@
+<?php
+
+require_once 'HTMLPurifier/AttrDef/Class.php';
+require_once 'HTMLPurifier/Config.php';
+
+class HTMLPurifier_AttrDef_ClassTest extends UnitTestCase
+{
+    
+    function testDefault() {
+        
+        $def = new HTMLPurifier_AttrDef_Class();
+        
+        $this->assertTrue($def->validate('valid'));
+        $this->assertTrue($def->validate('a0-_'));
+        $this->assertTrue($def->validate('-valid'));
+        $this->assertTrue($def->validate('_valid'));
+        $this->assertTrue($def->validate('double valid'));
+        
+        $this->assertFalse($def->validate('0invalid'));
+        $this->assertFalse($def->validate('-0'));
+        
+        // test conditional replacement
+        $this->assertEqual('validassoc', $def->validate('validassoc 0invalid'));
+        
+        // test whitespace leniency
+        $this->assertTrue('double valid', $def->validate(" double\nvalid\r"));
+        
+    }
+    
+}
+
+?>
\ No newline at end of file
diff --git a/tests/HTMLPurifier/Strategy/ValidateAttributesTest.php b/tests/HTMLPurifier/Strategy/ValidateAttributesTest.php
index 54f92c47..e59c3927 100644
--- a/tests/HTMLPurifier/Strategy/ValidateAttributesTest.php
+++ b/tests/HTMLPurifier/Strategy/ValidateAttributesTest.php
@@ -19,20 +19,22 @@ class HTMLPurifier_Strategy_ValidateAttributesTest extends
         $inputs[0] = '';
         $expect[0] = '';
         
+        // test ids
+        
         $inputs[1] = '<div id="valid">Preserve the ID.</div>';
         $expect[1] = $inputs[1];
         
         $inputs[2] = '<div id="0invalid">Kill the ID.</div>';
         $expect[2] = '<div>Kill the ID.</div>';
         
-        // test accumulator
+        // test id accumulator
         $inputs[3] = '<div id="valid">Valid</div><div id="valid">Invalid</div>';
         $expect[3] = '<div id="valid">Valid</div><div>Invalid</div>';
         
         $inputs[4] = '<span dir="up-to-down">Bad dir.</span>';
         $expect[4] = '<span>Bad dir.</span>';
         
-        // test case sensitivity
+        // test attribute case sensitivity
         $inputs[5] = '<div ID="valid">Convert ID to lowercase.</div>';
         $expect[5] = '<div id="valid">Convert ID to lowercase.</div>';
         
@@ -46,6 +48,10 @@ class HTMLPurifier_Strategy_ValidateAttributesTest extends
         $config[7] = HTMLPurifier_Config::createDefault();
         $config[7]->attr_id_blacklist = array('invalid');
         
+        // test classes
+        $inputs[8] = '<div class="valid">Valid</div>';
+        $expect[8] = $inputs[8];
+        
         $this->assertStrategyWorks($strategy, $inputs, $expect, $config);
         
     }
diff --git a/tests/index.php b/tests/index.php
index f6adc716..87196bb3 100644
--- a/tests/index.php
+++ b/tests/index.php
@@ -39,6 +39,7 @@ $test->addTestFile('HTMLPurifier/Strategy/CoreTest.php');
 $test->addTestFile('HTMLPurifier/Strategy/ValidateAttributesTest.php');
 $test->addTestFile('HTMLPurifier/AttrDef/EnumTest.php');
 $test->addTestFile('HTMLPurifier/AttrDef/IDTest.php');
+$test->addTestFile('HTMLPurifier/AttrDef/ClassTest.php');
 $test->addTestFile('HTMLPurifier/IDAccumulatorTest.php');
 $test->addTestFile('HTMLPurifier/TagTransformTest.php');