[robocat]

> They can only select at most one element.

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.

[jfrunyon]

Incorrect. getElementById returns a single element, regardless of how many elements you put with the same id; furthermore, having multiple elements with the same id is a violation of the relevant standards, so browsers may do whatever they want at that point (including ignore the repeated id on ALL elements, ignoring it on SOME elements, or honoring it on all elements). And in fact various browsers at various times have done all 3 of those things. Even today both Chrome and Firefox do two of those things depending on how you look at it (getElementById returns a single element, but CSS selectors apply to all elements with the ID).

It is only convention that gives anything meaning. HTML attributes are given meaning by convention. The words I'm writing right now are given meaning by convention. That implies abolutely nothing about the meaning or validity thereof.

> the id attribute value must be unique amongst all the IDs in the element's tree ... The id attribute specifies its element's unique identifier (ID).

https://html.spec.whatwg.org/multipage/dom.html#global-attri...