validation-example-tuned.js

    // ...

    it("reports error on invalid email", function(done) {
        client
            .setValue('#email', 'invalid@e.mail!');
            .click('#register');
            .getAttribute('#email.validated', 'class', function(err, result) {
                expect(result).to.include('error');
            })
            .call(done);
    });

    // ...

validation-example.js

    // ...

    it("reports error on invalid email", function(done) {
        client
            .setValue('#email', 'invalid@e.mail!');
            .click('#register');
            .getAttribute('#email', 'class', function(err, result) {
                expect(result).to.include('error');
            })
            .call(done);
    });

    // ...