diff --git a/mozilla/layout/tools/CSSProps.txt b/mozilla/layout/tools/CSSProps.txt
index 46fb0b04097..a15822976c2 100644
--- a/mozilla/layout/tools/CSSProps.txt
+++ b/mozilla/layout/tools/CSSProps.txt
@@ -59,6 +59,7 @@ margin-bottom
 margin-left
 margin-right
 margin-top
+opacity
 overflow
 padding
 padding-bottom