html - Does ID have to be unique in the whole page?

Question

I am using jQuery and I am just wondering, does ID have to be always unique in the whole page? Class, I know, can be repeated as many times as you like, what about ID?

Answer 1

Yes, it must be unique.

HTML4:

https://www.w3.org/TR/html4/struct/global.html#h-7.5.2

Section 7.5.2:

id = name [CS] This attribute assigns a name to an element. This name must be unique in a document.

HTML5:

https://www.w3.org/TR/html5/dom.html#element-attrdef-global-id

The id attribute specifies its element's unique identifier (ID). The value must be unique amongst all the IDs in the element's home subtree and must contain at least one character. The value must not contain any space characters.