Can multiple different HTML elements have the same ID if they're different elements?

Question

Can multiple HTML elements have the same ID if they\'re of different element types? Is a scenario like this valid? Eg:

div#foo
span#foo
a#foo

Answer 1

I think there is a difference between whether something SHOULD be unique or MUST be unique (i.e. enforced by web browsers).

Should IDs be unique? YES.

Must IDs be unique? NO, at least IE and FireFox allow multiple elements to have the same ID.