diff --git a/html/semantics/scripting-1/the-script-element/script-runs-on-character-data-mutation.window.js b/html/semantics/scripting-1/the-script-element/script-runs-on-character-data-mutation.window.js
new file mode 100644
index 00000000000000..8027a84a035950
--- /dev/null
+++ b/html/semantics/scripting-1/the-script-element/script-runs-on-character-data-mutation.window.js
@@ -0,0 +1,36 @@
+// https://github.com/whatwg/html/issues/12279
+test(() => {
+  window.script_did_run = false;
+
+  const script = document.createElement('script');
+
+  // This prevents execution on insertion.
+  script.type = '0';
+
+  const text = document.createTextNode('script_did_run = true;');
+  script.append(text);
+  document.body.append(script);
+  assert_false(script_did_run,
+      'Appending script with invalid type does not trigger execution');
+
+  // This enables, but does not trigger, execution.
+  script.type = '';
+  assert_false(script_did_run,
+      'Unsetting script type does not trigger execution');
+
+  text.data = 'script_did_run = true;';
+  assert_false(script_did_run,
+      'Setting CharacterData.data does not trigger execution');
+
+  text.appendData(' /* still should not run */');
+  assert_false(script_did_run,
+      'appendData() does not trigger execution');
+
+  text.replaceData(0, text.length, 'script_did_run = true;');
+  assert_false(script_did_run,
+      'replaceData() does not trigger execution');
+
+  text.deleteData(text.length - 1, 1);
+  assert_false(script_did_run,
+      'deleteData() does not trigger execution');
+}, "Script execution is never triggered on child mutation");