Automated coding test for front-end developers?

I'm looking for an automated coding test for front-end developers.

Before we[1] bring developers on-site for an interview we like to make sure they have some real coding skills. In the past we've used HackerRank, which has automated coding tests, but the problems (and languages) are not really appropriate for what a front-end developer does. I want to see them manipulate the DOM, do some CSS, ideally do some AJAX.

Is there a service or site that solves this problem? What do other companies in a similar situation do?

Bonus points if the site itself has a nice design, so people with design skills will take us seriously.

[1] http://BeakerNotebook.com, part of http://TwoSigma.com