1 files changed, 25 insertions, 0 deletions
diff --git a/testing/web-platform/tests/resources/declarative-shadow-dom-polyfill.js b/testing/web-platform/tests/resources/declarative-shadow-dom-polyfill.js
new file mode 100644
index 0000000000..99a3e911eb
--- /dev/null
+++ b/testing/web-platform/tests/resources/declarative-shadow-dom-polyfill.js
@@ -0,0 +1,25 @@
+/*
+ * Polyfill for attaching shadow trees for declarative Shadow DOM for
+ * implementations that do not support declarative Shadow DOM.
+ *
+ * Note: this polyfill will feature-detect the native feature, and do nothing
+ *       if supported.
+ *
+ * See: https://github.com/whatwg/html/pull/5465
+ *
+ * root: The root of the subtree in which to upgrade shadow roots
+ *
+ */
+
+function polyfill_declarative_shadow_dom(root) {
+  if (HTMLTemplateElement.prototype.hasOwnProperty('shadowRootMode'))
+    return;
+  root.querySelectorAll("template[shadowrootmode]").forEach(template => {
+    const mode = template.getAttribute("shadowrootmode");
+    const delegatesFocus = template.hasAttribute("shadowrootdelegatesfocus");
+    const shadowRoot = template.parentNode.attachShadow({ mode, delegatesFocus });
+    shadowRoot.appendChild(template.content);
+    template.remove();
+    polyfill_declarative_shadow_dom(shadowRoot);
+  });
+}