It’s a common CSS wisdom to use ids very sparingly because there’s always a chance you may want to have the same element on the page multiple times.
There are some scenarios in which this is the case you may not expect right away, sometimes you need to duplicate elements for responsivity, for example.
I think for CSS the reason is mostly that IDs have very high specificity, so styling you apply there is very hard to override elsewhere in your stylesheet.
However, that just means that you will probably generally want to avoid using IDs in your selectors - not to avoid using them altogether, or in JavaScript.
They should have high specificity - and it should be hard to override them with other selectors - because they are specific. They can only select at most one element. Why would you want to override it? Then you'd have cruft (i.e. a bug in waiting) in the obvious place to look in the future.
Incorrect. An ID can be repeated on as many elements in a page as you like. It is only convention that makes an id “unique”. I am not suggesting you do it, since here be dragons e.g. I just got a stack trace in the console of jsbin.com on Mobile Safari when I was testing with two elements with the same id — it breaks developer’s assumptions.
It is uncommon knowledge that id’s do not have to be unique.
document.querySelectorAll('#test').length will return 2 if there are two elements on the page with the same id=test attribute. I personally avoid using that ‘feature’ because it would confuse many people, but it is important to know if you run into certain bugs in code.
