Moving presentation (CSS) out of markup (HTML) can have a big impact on both end users and the server. Here’s an example from The Zen of CSS Design by Dave Shea and Molly E. Holzschlag:
When your traffic is high, the extra bytes trimmed have huge implications for your total bandwidth savings. Sports network ESPN’s web site went CSS in early 2003, and its numbers are almost unreal. The developers of ESPN.com estimate they’re saving about 2 terabytes (2 thousand gigabytes) each and every single day. This means they can serve more users, and serve them quicker, without changing anything else. That’s substantial return on a one-time investment.