diff --git a/interfaces/css-typed-om.idl b/interfaces/css-typed-om.idl
index 02fe7bb83ad7dc..be2946bc20f768 100644
--- a/interfaces/css-typed-om.idl
+++ b/interfaces/css-typed-om.idl
@@ -37,7 +37,7 @@ partial interface CSSStyleRule {
     [SameObject] readonly attribute StylePropertyMap styleMap;
 };
 
-partial interface ElementCSSInlineStyle {
+partial interface mixin ElementCSSInlineStyle {
     [SameObject] readonly attribute StylePropertyMap attributeStyleMap;
 };