diff --git a/library/HTMLPurifier.includes.php b/library/HTMLPurifier.includes.php
index f4956fa4..88ad66d3 100644
--- a/library/HTMLPurifier.includes.php
+++ b/library/HTMLPurifier.includes.php
@@ -136,6 +136,7 @@ require 'HTMLPurifier/ConfigSchema/Validator.php';
 require 'HTMLPurifier/ConfigSchema/Validator/Alnum.php';
 require 'HTMLPurifier/ConfigSchema/Validator/Composite.php';
 require 'HTMLPurifier/ConfigSchema/Validator/Exists.php';
+require 'HTMLPurifier/ConfigSchema/Validator/If.php';
 require 'HTMLPurifier/ConfigSchema/Validator/NamespaceExists.php';
 require 'HTMLPurifier/ConfigSchema/Validator/Or.php';
 require 'HTMLPurifier/ConfigSchema/Validator/ParseDefault.php';
diff --git a/library/HTMLPurifier/ConfigSchema/Interchange.php b/library/HTMLPurifier/ConfigSchema/Interchange.php
index 31e28775..58df5d70 100644
--- a/library/HTMLPurifier/ConfigSchema/Interchange.php
+++ b/library/HTMLPurifier/ConfigSchema/Interchange.php
@@ -60,7 +60,7 @@ class HTMLPurifier_ConfigSchema_Interchange
         $directive->addValidator($this->make('Alnum', '_DIRECTIVE'));
         $directive->addValidator($this->make('NamespaceExists'));
         
-        // Directive: Type tests
+        // Directive: Type and Default tests
         $directive->addValidator($this->make('Exists', 'TYPE'));
         $directive->addValidator($this->make('ParseType'));
         $directive->addValidator($this->make('Exists', '_TYPE'));
diff --git a/library/HTMLPurifier/ConfigSchema/Validator/If.php b/library/HTMLPurifier/ConfigSchema/Validator/If.php
new file mode 100644
index 00000000..470fb802
--- /dev/null
+++ b/library/HTMLPurifier/ConfigSchema/Validator/If.php
@@ -0,0 +1,49 @@
+<?php
+
+/**
+ * If a validator passes, run another validator.
+ */
+class HTMLPurifier_ConfigSchema_Validator_If extends HTMLPurifier_ConfigSchema_Validator
+{
+    
+    protected $condition;
+    protected $then;
+    protected $else;
+    
+    public function __construct($cond = null) {
+        $this->setCondition($cond);
+    }
+    
+    /**
+     * @param $validator Validator to run as a condition. Exceptions thrown by it
+     *        do not bubble up.
+     */
+    public function setCondition($validator) {
+        $this->condition = $validator;
+    }
+    
+    /**
+     * @param $validator Validator to run if condition is true
+     */
+    public function setThen($validator) {
+        $this->then = $validator;
+    }
+    
+    /**
+     * @param $validator Validator to run if condition is false
+     */
+    public function setElse($validator) {
+        $this->else = $validator;
+    }
+    
+    public function validate(&$arr, $interchange) {
+        try {
+            $this->condition->validate($arr, $interchange);
+        } catch (HTMLPurifier_ConfigSchema_Exception $e) {
+            if ($this->else) $this->else->validate($arr, $interchange);
+            return;
+        }
+        if ($this->then) $this->then->validate($arr, $interchange);
+    }
+    
+}
diff --git a/tests/HTMLPurifier/ConfigSchema/Validator/IfTest.php b/tests/HTMLPurifier/ConfigSchema/Validator/IfTest.php
new file mode 100644
index 00000000..73dbdb34
--- /dev/null
+++ b/tests/HTMLPurifier/ConfigSchema/Validator/IfTest.php
@@ -0,0 +1,31 @@
+<?php
+
+class HTMLPurifier_ConfigSchema_Validator_IfTest extends HTMLPurifier_ConfigSchema_ValidatorHarness
+{
+    
+    public function setup() {
+        parent::setup();
+        generate_mock_once('HTMLPurifier_ConfigSchema_Validator');
+    }
+    
+    public function testValidate() {
+        $arr = array('ID' => 'RD');
+        $this->validator->setCondition(new HTMLPurifier_ConfigSchema_Validator_Exists('ID'));
+        $this->validator->setThen($mock1 = new HTMLPurifier_ConfigSchema_ValidatorMock());
+        $mock1->expectOnce('validate', array($arr, $this->interchange));
+        $this->validator->setElse($mock2 = new HTMLPurifier_ConfigSchema_ValidatorMock());
+        $mock2->expectNever('validate');
+        $this->validator->validate($arr, $this->interchange);
+    }
+    
+    public function testValidateConditionIsFalse() {
+        $arr = array('ID' => 'RD');
+        $this->validator->setCondition(new HTMLPurifier_ConfigSchema_Validator_Exists('ALTID'));
+        $this->validator->setThen($mock1 = new HTMLPurifier_ConfigSchema_ValidatorMock());
+        $mock1->expectNever('validate');
+        $this->validator->setElse($mock2 = new HTMLPurifier_ConfigSchema_ValidatorMock());
+        $mock2->expectOnce('validate', array($arr, $this->interchange));
+        $this->validator->validate($arr, $this->interchange);
+    }
+    
+}