--- a/testing/web-platform/tests/docs/writing-tests/testharness-api.md
+++ b/testing/web-platform/tests/docs/writing-tests/testharness-api.md
@@ -175,18 +175,22 @@ promise_test(function() {
     });
 }, "Simple example");
 ```
 
 In the example above, `foo()` returns a Promise that resolves with the string
 "foo". The `test_function` passed into `promise_test` invokes `foo` and attaches
 a resolve reaction that verifies the returned value.
 
-Note that in the promise chain constructed in `test_function` assertions don't
-need to be wrapped in `step` or `step_func` calls.
+Note that in the promise chain constructed in `test_function`
+assertions don't need to be wrapped in `step` or `step_func`
+calls. However when mixing event handlers and `promise_test`, the
+event handler callback functions *do* need to be wrapped since an
+exception in these functions does not cause the promise chain to
+reject.
 
 Unlike Asynchronous Tests, Promise Tests don't start running until after the
 previous Promise Test finishes. [Under rare
 circumstances](https://github.com/web-platform-tests/wpt/pull/17924), the next
 test may begin to execute before the returned promise has settled. Use
 [add_cleanup](#cleanup) to register any necessary cleanup actions such as
 resetting global state that need to happen consistently before the next test
 starts.