Using the @media rule in your CSS allows you to target different media types, and screen sizes, from a single stylesheet. Using media queries with max-widths is integral to the current push towards responsive design. This can also be used to create your print styles using @media print. You'll notice this used on modern base-templates such as the HTML5 BoilerPlate:
While managing a few projects that were built by freelancers, I came up with a list of guidelines for the build and for WordPress used as a CMS. I've added reminders to it based upon the mistakes that fly in the face of what I consider a "good" build. Note that some of this is a matter of preference and opinion. I thought I would put this out there as a starting point for anyone trying to put together their own guidelines...
On July 23rd, 2010, I attended the Front-End Design Conference; "A single day design conference focused on content, presentation and behavior". The all-female lineup spoke to a packed room at the Hilton Bayfront in St. Petersburg, FL about branding, ideas, design, CSS, and Old Spice commercials. Sarah Parmenter even flew over from across the ocean to speak about designing apps for the iPhone...