Ask HN: Your coolest, most useful web design/dev tip?

I've found that web design, and back end development, is surrounded by an amazing community of people. With so many people doing neat things it's no wonder that so many nifty tips and tricks come about. Many of these tips or tricks become part of many people's bag-o-tricks - their "go to" way of achieving neat effects, or awesome typography, or just anything that makes web development that much more appealing. Often times it could be a certain font that you use at a certain size, and it just looks amazing you think. Or, some CSS trick that you feel gives your pages that current, trendy look. Even could be some Photoshop trick that you like to do with images for your sites. Anything. Most people have a few.

So, I thought it'd be fun to share yours. If you had to pick one or two items from your arsenal of tips/tricks, which would they be?