The only time I've known that the uniqueness of ids to be in play is either as a target of an anchor or to be used in Javascript. As far as I know, in those cas

The only time I've known that the uniqueness of ids to be in play is either as a target of an anchor or to be used in Javascript. As far as I know, in those cases only the first element with that id will be involved. Removing the CSS from your example, and then using Javascript to change the color you see that only the first one is altered. http://jsfiddle.net/KQkJE/1/ http://jsfiddle.net/KQkJE/1/ So, in terms of CSS, it doesn't really matter. Well, I don't know about performance issues with it though. But for Javascript it matters a great deal. As for HTML, it only really matters in that it's supposed to be unique for anchor targets, which would be one reason it doesn't validate. Browsers give a great deal of leeway when it comes to HTML and CSS, because so many people and editors do not follow the spec. Of course, you could argue that no one follows the spec because the browsers don't really enforce it. I've been constantly amazed over the years seeing various examples of HTML soup that should not render in any recognizable form that somehow renders in the browser.