In a perfect world, you’d never put anything in your HTML that defines the presentation of your page. You’d only use HTML for content, and allow the stylesheet to define the presentation. One of my few gripes with CSS is the inability to create a box with rounded corners. In general, CSS is pretty weak when it comes to defining custom borders. (How many drop shadow techniques could we kill if CSS allowed border images?).
I ran across several good examples today here and here of ways to make divs with rounded corners, although these same techniques could really be used to create whatever sort of custom border you like. Good articles to stick under the ol’ developer hat for later.