diff --git a/testing/web-platform/tests/html/semantics/scripting-1/the-script-element/change-src-attr-prepare-a-script.html b/testing/web-platform/tests/html/semantics/scripting-1/the-script-element/change-src-attr-prepare-a-script.html
new file mode 100644
index 0000000000000..9dc3808c90ca1
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+++ b/testing/web-platform/tests/html/semantics/scripting-1/the-script-element/change-src-attr-prepare-a-script.html
@@ -0,0 +1,42 @@
+<!doctype html>
+<meta charset=utf-8>
+<link rel=help href=https://github.com/whatwg/html/pull/10188#discussion_r1719338657>
+<title>Adding/changing src attribute does "prepare the script"</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<body>
+<script>
+// The "old" HTML Standard specification text around `src` attribute mutation
+// for non-parser-inserted scripts *ONLY* "prepared"/run the script iff the src
+// attribute "previously had no such attribute". This changed in
+// https://github.com/whatwg/html/pull/10188 to align with a majority of
+// browsers. This test ensures that `src` mutations on these kinds of scripts
+// *where a previous valid `src` attribute existed* do indeed "prepare" the
+// script.
+promise_test(async () => {
+  window.didExecute = false;
+
+  const script = document.createElement('script');
+  // Invalid type, so the script won't execute upon insertion.
+  script.type = 'invalid';
+  script.src = 'resources/flag-setter.js';
+  document.body.append(script);
+  assert_false(window.didExecute);
+
+  // Make script valid, but don't immediately execute it.
+  script.type = '';
+
+  const scriptPromise = new Promise(resolve => {
+    script.onload = resolve;
+  });
+
+  // Mutating the `src` attribute, which has an existing valid value, triggers
+  // the "prepare a script" algorithm via the post-connection steps.
+  script.src = 'resources/flag-setter.js?different';
+
+  await scriptPromise;
+  assert_true(window.didExecute);
+}, "Mutating `src` attribute from an already-valid value does 'prepare' the script");
+</script>
+</body>
