I think it's useful to consider things beyond speed. Unless your users can "really" tell the difference between the different selector speeds, you might want to consider the overall readability and maintainability. I am a big believer in "striving" towards Semantic Markup. This means that lists of items are in UL/LI structures and H1 represents your main page title etc etc. And your classes/ids/markup make sense to someone who is new and working on the CSS.
I try to make my markup (and fail miserably all too often) without class or ID. When I need something, I ask myself, "Is this something that is totally and uttery unique on the page. If yes, *ID*, if its questionable, *class.* And then I try to keep everything inside that node clean of id/class. Example: A global navigation and a local side navigation. They couldd both be <UL><LI> structures. Let's say you use ID's for the two <UL> so you can tell the difference. Then your CSS could style them differently. And you can bind events that could walk the DOM tree the same way, despite the fact that they are visually different. Anyway, I'd sure I am not explaining this well enough, but I really hate when half the css is based on classes and the other half based on IDs. I hate when half the jQuery is based on classes and the other half based on IDs. Again, if your users CAN tell the difference without a profiler, that is one thing. If they can't, then think about what is easiest to read and maintain. Please dont flame me everyone. :) Glen
