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Comment by simonw | Hacker News Reader

The problem with absolutely unique elements is that, in a complex enough website (that evolves over time), they often end up not being unique after all.

I agree about IDs for JavaScript though.

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Exactly, as far as CSS is concerned an id is effectively equivalent to a class name except when it breaks things.

Ids should be used only for scripts, and most of the time should be auto generated anyway.

It is not equivalent in any sense. Let's not forget specificity.

If you're relying on weird specificity chains, then your CSS is likely an unmaintainable mess.

Leave IDs for uniquely identifying an element on a page because you need to access it from a script.

Treating them like "Extra Strong" class names is a maintenance nightmare.

"I agree about IDs for JavaScript though."

Would you mind elaborating on the JavaScript point? Is it for convenience with document.getElementById or something?

Using jQuery, I tend to prefer class selectors the same as in my CSS.

There's less of a win in ajax from applying functionality on a class level - there aren't typically multiple selector boxes on the same page that all hit the same ajax callback.

Jquery class selector is the slowest selector, avoid them as much as possible. ID selector is the fastest. So, if your application is jquery intense, better avoid css selectors.

in modern querySelector implementations (any real browser), class look-ups are just as fast as IDs.

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