 <p>If you'd <a href="https://developer.mozilla.org/en-US/docs/CSS/Writing_Efficient_CSS" rel="nofollow">use only id's the selection would be faster</a>, but you would also have to do more selections than if you would use classes. Provided that the css is clean. Problem with basing style on id's in general is that you forgo all reuse of that style.</p> <p>The best way to go is to link style to classes. That gives you adequate speed and keeps the css file smaller. Smaller files are much more important for performance in most sites than faster execution on the client. Typical clients will render a complex page in microseconds, once they have all the resources that is.</p> <p>The css you point out is using a sprite, which could very well be generated with compass or something similar. It's important to see the original code too before making harsh judgements about the quality.</p>