LayoutTests/fast/forms/validation-custom-message.html

<!DOCTYPE html>
<html>
<body>
<script src="../../resources/js-test.js"></script>
<script src="form-validation.js"></script>
<form>
  Required text input: <input type="text" id="required_text_input" required><input id="required_text_input_submit" type="submit">
</form>
<div id="description"></div>
<div id="console"></div>
<script>
description("Tests that the custom validation message set by JavaScript is displayed on UI side.");
jsTestIsAsync = true;

onload = function() {
    input = document.getElementById("required_text_input");
    input.setCustomValidity("This is a custom validity message.");
    submit = document.getElementById("required_text_input_submit");
    submit.click();

    getValidationMessage().then((_message) => {
        message = _message;
        shouldBeEqualToString("message", "This is a custom validity message.");

        // Reset the validity message to the default one.
        input.setCustomValidity("");
        submit.click();

        getValidationMessage().then((_message) => {    
            message = _message;
            shouldBeEqualToString("message", "Fill out this field");
            finishJSTest();
        });
    });
}
</script>
</body>
<html>