diff --git a/testing/web-platform/tests/interfaces/construct-stylesheets.idl b/testing/web-platform/tests/interfaces/construct-stylesheets.idl
index fca2a3e2887a..35121866f66e 100644
--- a/testing/web-platform/tests/interfaces/construct-stylesheets.idl
+++ b/testing/web-platform/tests/interfaces/construct-stylesheets.idl
@@ -16,6 +16,6 @@ dictionary CSSStyleSheetInit {
   boolean disabled = false;
 };
 
-partial interface DocumentOrShadowRoot {
+partial interface mixin DocumentOrShadowRoot {
   attribute FrozenArray<CSSStyleSheet> adoptedStyleSheets;
 };