In “Jeffrey Zeldman: King of Web Standards,” Jessie Scanlon explains how CSS works:
CSS allowed developers to separate content from appearance; style sheets are like little notes that say to the Web server, “If you’re sending a page to a PC, make it look like this.” There might be separate sheets for PCs, for a “printer-friendly” layout, for a PDA, and so on. For designers, CSS means that the page will appear as it was intended, no matter what the device. For developers, CSS means they only have to build the page once. And for users, CSS means, as Zeldman says, that the site works.
For companies with a Web presence—needless to say, most companies—CSS means “You can control you branding, your image, and still deliver content to users in the most appropriate style,” Zeldman says. It also means that a site redesign wouldn’t require every page to be recoded—an expensive and time-consuming proposition.