diff --git a/NEWS b/NEWS
index e6e0b9b0..952b6e73 100644
--- a/NEWS
+++ b/NEWS
@@ -10,6 +10,8 @@ NEWS ( CHANGELOG and HISTORY )                                     HTMLPurifier
 ==========================
 
 4.7.0, unknown release date
+# opacity is now considered a "tricky" CSS property rather than a
+  proprietary one.
 - Don't truncate upon encountering </div> when using DOMLex.  Thanks
   Myrto Christina for finally convincing me to fix this.
 
diff --git a/library/HTMLPurifier/CSSDefinition.php b/library/HTMLPurifier/CSSDefinition.php
index 0acdee2d..07cc9417 100644
--- a/library/HTMLPurifier/CSSDefinition.php
+++ b/library/HTMLPurifier/CSSDefinition.php
@@ -350,8 +350,7 @@ class HTMLPurifier_CSSDefinition extends HTMLPurifier_Definition
         $this->info['scrollbar-highlight-color'] = new HTMLPurifier_AttrDef_CSS_Color();
         $this->info['scrollbar-shadow-color'] = new HTMLPurifier_AttrDef_CSS_Color();
 
-        // technically not proprietary, but CSS3, and no one supports it
-        $this->info['opacity'] = new HTMLPurifier_AttrDef_CSS_AlphaValue();
+        // vendor specific prefixes of opacity
         $this->info['-moz-opacity'] = new HTMLPurifier_AttrDef_CSS_AlphaValue();
         $this->info['-khtml-opacity'] = new HTMLPurifier_AttrDef_CSS_AlphaValue();
 
@@ -404,6 +403,7 @@ class HTMLPurifier_CSSDefinition extends HTMLPurifier_Definition
             array('visible', 'hidden', 'collapse')
         );
         $this->info['overflow'] = new HTMLPurifier_AttrDef_Enum(array('visible', 'hidden', 'auto', 'scroll'));
+        $this->info['opacity'] = new HTMLPurifier_AttrDef_CSS_AlphaValue();
     }
 
     /**