DOM: CEReactions vs script during insertion

Complements #15264.

dom/nodes/Node-appendChild-cereactions-vs-script.window.js

@@ -0,0 +1,27 @@
+const results = [];
+test(() => {
+  class Script1 extends HTMLScriptElement {
+    constructor() {
+      super();
+    }
+    connectedCallback() {
+      results.push("ce connected s1");
+    }
+  }
+  class Script2 extends HTMLScriptElement {
+    constructor() {
+      super();
+    }
+    connectedCallback() {
+      results.push("ce connected s2");
+    }
+  }
+  customElements.define("script-1", Script1, { extends: "script" });
+  customElements.define("script-2", Script2, { extends: "script" });
+  const s1 = new Script1();
+  s1.textContent = "results.push('s1')";
+  const s2 = new Script2();
+  s2.textContent = "results.push('s2')";
+  document.body.append(s1, s2);
+  assert_array_equals(results, ["s1", "s2", "ce connected s1", "ce connected s2"]);
+}, "Custom element reactions follow script execution");