Q:

How do you test visual components?

I think I understand unit testing.  But I was wondering: is there a way to automate something visual, like making sure anti-aliasing works or that the rounded corners on a web site look how they are supposed to?  I have a feeling that it just isn't practical, but I have little experience in the QA world.

A:

"anti-aliasing works or that the rounded corners on a web site look how they are supposed to?"

You could theoretically write automated tools to take snapshots of the renderings and somehow parse them and compare vs the mockup. 
You'd need some sort of "anchoring" mechanism to align the model answer with the rendered output and then do a colourwise diff on it, where the resulting output being a complete black image == perfect compliance. 
However, doing this is undoubtedly exhaustively complicated, and wont compensate for perceptual failures such as how it is seen to a colourblind person on a screen with a poorly calibrated colourmap. 
What you instead need to do is create a full tree of all navigation paths possible in the application ( a little easier to do programmatically/test suite driven ), and then hand a specification to a set of humans to go through and execute on a variety of platforms. 

That will handle this week, but will
  you repeat the manual tests evey time
  you make a change to code? Or every
  time you make a release? Manual tests
  simply will not scale. They're cheap
  in the short, but hopelessly costly in
  the long run. – Tim Ottinger

FWIW, Even Firefox still has a human driven test suite.  Humans are simply better at recognising behaviour that while meeting the defined standard, does not meet some other standard that has not yet been arbitrarily defined, and thus, has not had a test-case written for it yet. 

