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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?
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The value of the id attribute must be unique within the HTML document. The id attribute is used to point to a specific style declaration in a style sheet. It is also used by JavaScript to access and manipulate the element with the specific id.
Remember: A webpage element has certain inalienable rights; having a unique ID is one of them. Prevent html element identity theft by ensuring that every element on your webpage that needs an ID has a unique one.
id 's value must not contain whitespace (spaces, tabs etc.). Browsers treat non-conforming IDs that contain whitespace as if the whitespace is part of the ID. In contrast to the class attribute, which allows space-separated values, elements can only have one single ID value.
An element can't have more than one ID and an ID can't be used more than once in a page.
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.
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Does an ID have to be unique in the whole page?
Because the HTML Living Standard of September 7, 2021, clearly states:
The
class,id, andslotattributes may be specified on all HTML elements. ……. When specified on HTML elements, theidattribute value must be unique amongst all the IDs in the element’s tree and must contain at least one character. The value must not contain any ASCII whitespace.
and a page may have several DOM trees. It does, for example, when you’ve attached (Element.attachShadow()) a shadow DOM tree to an element.
Does an ID have to be unique in the whole page?
No.
Does an ID have to be unique in a DOM tree?
Yes.
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