diff --git a/remote/shared/webdriver/Actions.sys.mjs b/remote/shared/webdriver/Actions.sys.mjs
index 7c87e5b11f2c..9d3e793541c6 100644
--- a/remote/shared/webdriver/Actions.sys.mjs
+++ b/remote/shared/webdriver/Actions.sys.mjs
@@ -766,17 +766,25 @@ class KeyAction extends Action {
   static fromJSON(id, actionItem) {
     const { value } = actionItem;
 
-    // TODO countGraphemes
-    // TODO key.value could be a single code point like "\uE012"
-    // (see rawKey) or "grapheme cluster"
-    // https://bugzilla.mozilla.org/show_bug.cgi?id=1496323
-
     lazy.assert.string(
       value,
       'Expected "value" to be a string that represents single code point ' +
         lazy.pprint`or grapheme cluster, got ${value}`
     );
 
+    let segmenter = new Intl.Segmenter();
+    lazy.assert.that(v => {
+      let graphemeIterator = segmenter.segment(v)[Symbol.iterator]();
+      // We should have exactly one grapheme cluster, so the first iterator
+      // value must be defined, but the second one must be undefined
+      return (
+        graphemeIterator.next().value !== undefined &&
+        graphemeIterator.next().value === undefined
+      );
+    }, `Expected "value" to be a string that represents single code point or grapheme cluster, got "${value}"`)(
+      value
+    );
+
     return new this(id, { value });
   }
 }
diff --git a/testing/web-platform/meta/webdriver/tests/bidi/input/perform_actions/invalid.py.ini b/testing/web-platform/meta/webdriver/tests/bidi/input/perform_actions/invalid.py.ini
deleted file mode 100644
index 344db3f7113f..000000000000
--- a/testing/web-platform/meta/webdriver/tests/bidi/input/perform_actions/invalid.py.ini
+++ /dev/null
@@ -1,19 +0,0 @@
-[invalid.py]
-  expected:
-    if (os == "linux") and not debug and not asan and not tsan: [OK, TIMEOUT]
-    if (os == "win") and not debug: [OK, TIMEOUT]
-  [test_params_key_action_value_invalid_multiple_codepoints[fa\]]
-    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1826808
-    expected: FAIL
-
-  [test_params_key_action_value_invalid_multiple_codepoints[\\u0ba8\\u0bbfb\]]
-    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1826808
-    expected: FAIL
-
-  [test_params_key_action_value_invalid_multiple_codepoints[\\u0ba8\\u0bbf\\u0ba8\]]
-    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1826808
-    expected: FAIL
-
-  [test_params_key_action_value_invalid_multiple_codepoints[\\u1100\\u1161\\u11a8c\]]
-    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1826808
-    expected: FAIL
diff --git a/testing/web-platform/tests/webdriver/tests/bidi/input/perform_actions/key_tentative.py b/testing/web-platform/tests/webdriver/tests/bidi/input/perform_actions/key_tentative.py
new file mode 100644
index 000000000000..1f7053c24997
--- /dev/null
+++ b/testing/web-platform/tests/webdriver/tests/bidi/input/perform_actions/key_tentative.py
@@ -0,0 +1,30 @@
+import pytest
+
+from webdriver.bidi.error import NoSuchFrameException
+from webdriver.bidi.modules.input import Actions
+from webdriver.bidi.modules.script import ContextTarget
+
+from tests.support.keys import Keys
+from .. import get_keys_value
+from . import get_shadow_root_from_test_page
+
+pytestmark = pytest.mark.asyncio
+
+@pytest.mark.parametrize(
+    "value",
+    [
+        ("\u0e01\u0e33"),
+        ("🤷🏽‍♀️"),
+    ],
+)
+async def test_grapheme_cluster(
+    bidi_session, top_context, setup_key_test, value
+):
+    actions = Actions()
+    (actions.add_key().key_down(value).key_up(value))
+    await bidi_session.input.perform_actions(
+        actions=actions, context=top_context["context"]
+    )
+    # events sent by major browsers are inconsistent so only check key value
+    keys_value = await get_keys_value(bidi_session, top_context["context"])
+    assert keys_value == value
diff --git a/testing/web-platform/tests/webdriver/tests/classic/perform_actions/invalid.py b/testing/web-platform/tests/webdriver/tests/classic/perform_actions/invalid.py
index f000477376e7..d83285abbe07 100644
--- a/testing/web-platform/tests/webdriver/tests/classic/perform_actions/invalid.py
+++ b/testing/web-platform/tests/webdriver/tests/classic/perform_actions/invalid.py
@@ -1,3 +1,4 @@
+# META: timeout=long
 import pytest
 
 from webdriver.error import InvalidArgumentException
@@ -304,6 +305,28 @@ def test_key_action_value_invalid_type(session, key_action, value):
     assert_error(response, "invalid argument")
 
 
+@pytest.mark.parametrize("action_type", ["keyDown", "keyUp"])
+@pytest.mark.parametrize(
+    "value",
+    ["fa", "\u0BA8\u0BBFb", "\u0BA8\u0BBF\u0BA8", "\u1100\u1161\u11A8c"],
+)
+def test_key_action_invalid_value(session, action_type, value):
+    actions = [
+        {
+            "type": "key",
+            "id": "foo",
+            "actions": [
+                {
+                    "type": action_type,
+                    "value": value,
+                }
+            ],
+        }
+    ]
+    response = perform_actions(session, actions)
+    assert_error(response, "invalid argument")
+
+
 @pytest.mark.parametrize("value", ["", "pointerDowns", "pointerMoves", "pointerUps"])
 def test_pointer_action_subtype_invalid_value(session, value):
     if value == "pointerMoves":
diff --git a/testing/web-platform/tests/webdriver/tests/classic/perform_actions/key_tentative.py b/testing/web-platform/tests/webdriver/tests/classic/perform_actions/key_tentative.py
new file mode 100644
index 000000000000..62f380da0ec0
--- /dev/null
+++ b/testing/web-platform/tests/webdriver/tests/classic/perform_actions/key_tentative.py
@@ -0,0 +1,15 @@
+import pytest
+
+from tests.classic.perform_actions.support.refine import get_keys
+
+@pytest.mark.parametrize("value",
+    [
+        ("\u0e01\u0e33"),
+        ("🤷🏽‍♀️"),
+    ])
+def test_grapheme_cluster(session, key_reporter, key_chain, value):
+    key_chain \
+        .key_down(value) \
+        .key_up(value) \
+        .perform()
+    assert get_keys(key_reporter) == value
diff --git a/tools/lint/trojan-source.yml b/tools/lint/trojan-source.yml
index 4f4263fc20c2..1238a6874dbb 100644
--- a/tools/lint/trojan-source.yml
+++ b/tools/lint/trojan-source.yml
@@ -21,6 +21,8 @@ trojan-source:
         - third_party/rust/error-chain/tests/tests.rs
         - third_party/rust/unicode-width/src/tests.rs
         - security/nss/gtests/mozpkix_gtest/pkixnames_tests.cpp
+        - testing/web-platform/tests/webdriver/tests/bidi/input/perform_actions/key_tentative.py
+        - testing/web-platform/tests/webdriver/tests/classic/perform_actions/key_tentative.py
         - toolkit/components/uniffi-fixtures/rondpoint/tests/bindings/test_rondpoint.py
     extensions:
         - .c