Zen and the Art of DOCTYPE switching.
Today's browser technology includes a means by which designers can gain more control over the consistency of their designs. Especially true if you're using CSS, knowing about DOCTYPE Switching in browsers will help you breathe easier, lose less hair, and get a decent night's sleep knowing your designs are as consistent across contemporary browsers as possible.
Molly Hozschlag walks you through the Zen of DOCTYPE switching, the Whys, the Hows and the Oms.