You look at it :-) (Edit: That's not the whole answer. Usually when you test you assert that certain elements of the DOM are where they should be. At the very least you have verified the structure of the HTML document, if not how it looks. For actual aesthetics I can't see any way around human testing.)

I am just playing devil's advocate.

I work for Fortune 500 consulting projects and don't believe in TDD for UI code that works across the myriad of technologies we use in work projects, while scaling across multi-site development.

TDD only seems to work on the business logic, and only in code written with testing in mind.

You have the computer look at it. Brett Slatkin of Google Consumer Surveys has a great talk on using perceptual diffs for this:

https://air.mozilla.org/continuous-delivery-at-google/