How does itemid identify a microdata item globally?

Definition: itemid is the Microdata attribute that supplies an item's global identifier as a URL. Its meaning comes from the vocabulary named by itemtype, and the attribute belongs on the same element as itemscope and itemtype when that vocabulary supports global identifiers.

Key facts

• The WHATWG Microdata specification defines itemscope as the attribute that creates an item, a group of name-value pairs.
• MDN's itemid reference describes itemid as a unique, global identifier for an item and requires the element to have both itemscope and itemtype.
• Schema.org's Microdata guide uses https://schema.org/Movie as an itemtype value and explains that item types are provided as URLs.

What does itemid identify?

Microdata describes an item as a group of name-value pairs. itemscope creates that item, while itemprop names properties on it. itemid adds an identifier for the item when its vocabulary defines support for global identifiers. The standard describes this as a way to relate the item to other items on pages elsewhere on the web.

The identifier is a URL value. It is associated with the item itself, rather than with one of its properties. A property such as itemprop="url" can describe a URL as a value inside the item, while itemid identifies the item that contains the properties.

Why are itemscope and itemtype required?

itemscope establishes the item to which the identifier belongs. itemtype supplies the item's type on that same element. The type gives the context for property names and selects a vocabulary, so the vocabulary can define what its itemid URLs mean.

The HTML Standard permits itemid when the element has both attributes and the vocabulary referenced by itemtype supports global identifiers. MDN states the same relationship and explains that the identifier's exact meaning comes from its definition in the specified vocabulary. A vocabulary can also define how several items with the same identifier coexist and how those items are processed.

What URL value does the browser use?

The standard requires a valid URL in itemid. It parses the attribute value relative to the node document of the element carrying the attribute. Therefore, the identifier used by the Microdata model is the parsed URL, not an unprocessed attribute string. If the attribute is missing or URL parsing fails, the item has no global identifier.

A minimal pattern is:

<div itemscope
     itemtype="https://example.org/book"
     itemid="https://example.org/books/identifier">
  <span itemprop="name">Book title</span>
</div>

Here, itemscope creates the item, itemtype names the vocabulary and type, itemid supplies the URL identifier, and itemprop supplies a property. The example uses an absolute URL so its value does not depend on the document URL.

How should you check the markup?

Check the element carrying itemid for itemscope and itemtype. Then check that the itemtype vocabulary supports global identifiers and defines the meaning of the URL. Finally, validate the value as a URL and account for relative URL resolution against the document that contains the element. These are separate checks because itemscope creates the item, itemtype supplies its vocabulary, and itemid supplies the identifier.

Related terms

• Microdata
• Structured data
• Schema.org @id
• JSON-LD

FAQ

Can itemid appear without itemscope?

No. The HTML Standard says that itemid must not be specified unless the element has both itemscope and itemtype, and the referenced vocabulary supports global identifiers.

Does every vocabulary assign the same meaning to an itemid URL?

No. The vocabulary specification determines the exact meaning of the global identifier and defines how repeated identifiers are handled. MDN documents the same vocabulary-dependent rule.

Is the literal attribute text the final identifier?

The Microdata model parses the itemid value as a URL relative to the element's node document. A missing value or a parsing failure leaves the item without a global identifier.

Sources

1. WHATWG, "HTML Standard: Microdata", updated 19 October 2026
2. MDN Web Docs, "itemid HTML global attribute", updated 17 April 2026
3. Schema.org, "Getting started with schema.org using Microdata", undated

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