diff --git a/library/HTMLPurifier/CSSDefinition.php b/library/HTMLPurifier/CSSDefinition.php
index 9ff09b78..72eb34fa 100644
--- a/library/HTMLPurifier/CSSDefinition.php
+++ b/library/HTMLPurifier/CSSDefinition.php
@@ -4,6 +4,7 @@ require_once 'HTMLPurifier/AttrDef/Enum.php';
 require_once 'HTMLPurifier/AttrDef/Color.php';
 require_once 'HTMLPurifier/AttrDef/Composite.php';
 require_once 'HTMLPurifier/AttrDef/CSSLength.php';
+require_once 'HTMLPurifier/AttrDef/Percentage.php';
 
 class HTMLPurifier_CSSDefinition
 {