diff --git a/NEWS b/NEWS
index c6e16ebf..35ace744 100644
--- a/NEWS
+++ b/NEWS
@@ -32,6 +32,7 @@ NEWS ( CHANGELOG and HISTORY )                                     HTMLPurifier
   of the operational document type
 . Lexer is now pre-emptively included, with a conditional include for the
   PHP5 only version.
+. HTMLDefinition and CSSDefinition have a common parent class: Definition.
 
 1.6.1, released 2007-05-05
 ! Support for more deprecated attributes via transformations:
diff --git a/library/HTMLPurifier/CSSDefinition.php b/library/HTMLPurifier/CSSDefinition.php
index 23a66ab7..4db04694 100644
--- a/library/HTMLPurifier/CSSDefinition.php
+++ b/library/HTMLPurifier/CSSDefinition.php
@@ -1,5 +1,7 @@
 <?php
 
+require_once 'HTMLPurifier/Definition.php';
+
 require_once 'HTMLPurifier/AttrDef/CSS/Background.php';
 require_once 'HTMLPurifier/AttrDef/CSS/BackgroundPosition.php';
 require_once 'HTMLPurifier/AttrDef/CSS/Border.php';
@@ -19,7 +21,7 @@ require_once 'HTMLPurifier/AttrDef/Enum.php';
  * Defines allowed CSS attributes and what their values are.
  * @see HTMLPurifier_HTMLDefinition
  */
-class HTMLPurifier_CSSDefinition
+class HTMLPurifier_CSSDefinition extends HTMLPurifier_Definition
 {
     
     /**
@@ -30,7 +32,7 @@ class HTMLPurifier_CSSDefinition
     /**
      * Constructs the info array.  The meat of this class.
      */
-    function setup($config) {
+    function doSetup($config) {
         
         $this->info['text-align'] = new HTMLPurifier_AttrDef_Enum(
             array('left', 'right', 'center', 'justify'), false);
diff --git a/library/HTMLPurifier/Config.php b/library/HTMLPurifier/Config.php
index 10db8245..38011a53 100644
--- a/library/HTMLPurifier/Config.php
+++ b/library/HTMLPurifier/Config.php
@@ -200,11 +200,11 @@ class HTMLPurifier_Config
                 $this->html_definition = HTMLPurifier_HTMLDefinition::getCache($this);
                 if ($this->html_definition) return $this->html_definition;
             }
-            $this->html_definition = new HTMLPurifier_HTMLDefinition($this);
+            $this->html_definition = new HTMLPurifier_HTMLDefinition();
             if ($raw) return $this->html_definition; // no setup!
         }
         if (!$this->html_definition->setup) {
-            $this->html_definition->setup();
+            $this->html_definition->setup($this);
             $this->html_definition->saveCache($this);
         }
         return $this->html_definition;
diff --git a/library/HTMLPurifier/HTMLDefinition.php b/library/HTMLPurifier/HTMLDefinition.php
index d75be453..5b66a871 100644
--- a/library/HTMLPurifier/HTMLDefinition.php
+++ b/library/HTMLPurifier/HTMLDefinition.php
@@ -1,6 +1,6 @@
 <?php
 
-// components
+require_once 'HTMLPurifier/Definition.php';
 require_once 'HTMLPurifier/HTMLModuleManager.php';
 
 // this definition and its modules MUST NOT define configuration directives
@@ -85,7 +85,7 @@ HTMLPurifier_ConfigSchema::define(
  * HTMLPurifier_Printer_HTMLDefinition is a notable exception to this
  * rule: in the interest of comprehensiveness, it will sniff everything.
  */
-class HTMLPurifier_HTMLDefinition
+class HTMLPurifier_HTMLDefinition extends HTMLPurifier_Definition
 {
     
     /** FULLY-PUBLIC VARIABLES */
@@ -155,17 +155,12 @@ class HTMLPurifier_HTMLDefinition
     
     /** PUBLIC BUT INTERNAL VARIABLES */
     
-    var $setup = false; /**< Has setup() been called yet? */
-    var $config; /**< Temporary instance of HTMLPurifier_Config */
-    
     var $manager; /**< Instance of HTMLPurifier_HTMLModuleManager */
     
     /**
      * Performs low-cost, preliminary initialization.
-     * @param $config Instance of HTMLPurifier_Config
      */
-    function HTMLPurifier_HTMLDefinition($config) {
-        $this->config = $config;
+    function HTMLPurifier_HTMLDefinition() {
         $this->manager = new HTMLPurifier_HTMLModuleManager();
     }
     
@@ -207,31 +202,18 @@ class HTMLPurifier_HTMLDefinition
         fclose($fh);
     }
     
-    /**
-     * Processes internals into form usable by HTMLPurifier internals. 
-     * Modifying the definition after calling this function should not
-     * be done.
-     */
-    function setup() {
-        
-        // multiple call guard
-        if ($this->setup) {return;} else {$this->setup = true;}
-        
-        $this->processModules();
-        $this->setupConfigStuff();
-        
-        // remove complicated variables to ease serialization
-        unset($this->config);
+    function doSetup($config) {
+        $this->processModules($config);
+        $this->setupConfigStuff($config);
         unset($this->manager);
-        
     }
     
     /**
      * Extract out the information from the manager
      */
-    function processModules() {
+    function processModules($config) {
         
-        $this->manager->setup($this->config);
+        $this->manager->setup($config);
         $this->doctype = $this->manager->doctype;
         
         foreach ($this->manager->modules as $module) {
@@ -257,9 +239,9 @@ class HTMLPurifier_HTMLDefinition
     /**
      * Sets up stuff based on config. We need a better way of doing this.
      */
-    function setupConfigStuff() {
+    function setupConfigStuff($config) {
         
-        $block_wrapper = $this->config->get('HTML', 'BlockWrapper');
+        $block_wrapper = $config->get('HTML', 'BlockWrapper');
         if (isset($this->info_content_sets['Block'][$block_wrapper])) {
             $this->info_block_wrapper = $block_wrapper;
         } else {
@@ -267,7 +249,7 @@ class HTMLPurifier_HTMLDefinition
                 E_USER_ERROR);
         }
         
-        $parent = $this->config->get('HTML', 'Parent');
+        $parent = $config->get('HTML', 'Parent');
         $def = $this->manager->getElement($parent, true);
         if ($def) {
             $this->info_parent = $parent;
@@ -283,7 +265,7 @@ class HTMLPurifier_HTMLDefinition
                    "support forums) ";
         
         // setup allowed elements, SubtractiveWhitelist module(?)
-        $allowed_elements = $this->config->get('HTML', 'AllowedElements');
+        $allowed_elements = $config->get('HTML', 'AllowedElements');
         if (is_array($allowed_elements)) {
             foreach ($this->info as $name => $d) {
                 if(!isset($allowed_elements[$name])) unset($this->info[$name]);
@@ -296,7 +278,7 @@ class HTMLPurifier_HTMLDefinition
             }
         }
         
-        $allowed_attributes = $this->config->get('HTML', 'AllowedAttributes');
+        $allowed_attributes = $config->get('HTML', 'AllowedAttributes');
         $allowed_attributes_mutable = $allowed_attributes; // by copy!
         if (is_array($allowed_attributes)) {
             foreach ($this->info_global_attr as $attr_key => $info) {
diff --git a/tests/HTMLPurifier/ConfigTest.php b/tests/HTMLPurifier/ConfigTest.php
index 61381731..5952d2f4 100644
--- a/tests/HTMLPurifier/ConfigTest.php
+++ b/tests/HTMLPurifier/ConfigTest.php
@@ -228,19 +228,23 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
         $this->old_copy = HTMLPurifier_ConfigSchema::instance($this->old_copy);
         
         $config = HTMLPurifier_Config::createDefault();
+        $config->set('HTML', 'Doctype', 'XHTML 1.0 Strict');
         $config->autoFinalize = false;
         
         $def = $config->getCSSDefinition();
         $this->assertIsA($def, 'HTMLPurifier_CSSDefinition');
         
-        $def = $config->getHTMLDefinition();
-        $def2 = $config->getHTMLDefinition();
+        $def  =& $config->getHTMLDefinition();
+        $def2 =& $config->getHTMLDefinition();
         $this->assertIsA($def, 'HTMLPurifier_HTMLDefinition');
-        $this->assertIdentical($def, $def2);
+        $this->assertReference($def, $def2);
         $this->assertTrue($def->setup);
         
         // test re-calculation if HTML changes
-        $config->set('HTML', 'Strict', true);
+        unset($def, $def2);
+        $def2 = $config->getHTMLDefinition(); // forcibly de-reference
+        
+        $config->set('HTML', 'Doctype', 'HTML 4.01 Transitional');
         $def = $config->getHTMLDefinition();
         $this->assertIsA($def, 'HTMLPurifier_HTMLDefinition');
         $this->assertNotEqual($def, $def2);