diff --git a/components/style/properties/data.py b/components/style/properties/data.py
index 8e6cdd79a1b..9ab3a40ca66 100644
--- a/components/style/properties/data.py
+++ b/components/style/properties/data.py
@@ -688,6 +688,7 @@ class PropertyRestrictions:
     def first_letter(data):
         props = set([
             "color",
+            "opacity",
             "float",
             "initial-letter",
 
@@ -722,6 +723,7 @@ class PropertyRestrictions:
         props = set([
             # Per spec.
             "color",
+            "opacity",
 
             # Kinda like css-fonts?
             "-moz-osx-font-smoothing",