diff --git a/parser/html/nsHtml5TreeOperation.cpp b/parser/html/nsHtml5TreeOperation.cpp
index 3877e01b89..57835bd36a 100644
--- a/parser/html/nsHtml5TreeOperation.cpp
+++ b/parser/html/nsHtml5TreeOperation.cpp
@@ -439,7 +439,9 @@ nsHtml5TreeOperation::CreateElement(int32_t aNs,
                           false);
 
       // Custom element setup may be needed if there is an "is" attribute.
-      if (kNameSpaceID_None == nsuri && !prefix && nsGkAtoms::is == localName) {
+        if (nsContentUtils::IsWebComponentsEnabled() &&
+            kNameSpaceID_None == nsuri &&
+            !prefix && nsGkAtoms::is == localName) {
         nsContentUtils::SetupCustomElement(newContent, &value);
       }
     }