new file mode 100644
--- /dev/null
+++ b/testing/web-platform/tests/xhr/send-data-string-invalid-unicode.any.js
@@ -0,0 +1,46 @@
+// META: title=XMLHttpRequest.send(invalidUnicodeString)
+
+const LEFT_SURROGATE = '\ud83d';
+const RIGHT_SURROGATE = '\udc94';
+
+// Unmatched surrogates should be replaced with the unicode replacement
+// character, 0xFFFD. '$' in these templates is replaced with one of
+// LEFT_SURROGATE or RIGHT_SURROGATE according to the test.
+const TEMPLATES = {
+  '$': [239, 191, 189],
+  '$ab': [239, 191, 189, 97, 98],
+  'a$b': [97, 239, 191, 189, 98],
+  'ab$': [97, 98, 239, 191, 189],
+};
+
+for (const surrogate of [LEFT_SURROGATE, RIGHT_SURROGATE]) {
+  for (const [template, expected] of Object.entries(TEMPLATES)) {
+    const invalidString = template.replace('$', surrogate);
+    const printableString = template.replace(
+        '$', '\\u{' + surrogate.charCodeAt(0).toString(16) + '}');
+    async_test(t => {
+      xhrSendStringAndCheckResponseBody(t, invalidString, expected);
+    }, `invalid unicode '${printableString}' should be fixed with ` +
+               `replacement character`);
+  }
+}
+
+// For the sake of completeness, verify that matched surrogates work.
+async_test(t => {
+  xhrSendStringAndCheckResponseBody(t, LEFT_SURROGATE + RIGHT_SURROGATE,
+                                    [240, 159, 146, 148]);
+}, 'valid unicode should be sent correctly');
+
+function xhrSendStringAndCheckResponseBody(t, string, expected) {
+  const xhr = new XMLHttpRequest();
+  xhr.responseType = 'arraybuffer';
+  xhr.onload = t.step_func(() => {
+    assert_equals(xhr.status, 200, 'status should be 200');
+    const actualBody = new Uint8Array(xhr.response);
+    assert_array_equals(actualBody, expected, 'content should match');
+    t.done();
+  });
+  xhr.onerror = t.unreached_func('no error should occur');
+  xhr.open('POST', 'resources/content.py', true);
+  xhr.send(string);
+}