HTML: Custom Data Attribute

By Xah Lee. Date: . Last updated: .

What is Custom Data Attribute

Custom Data Attribute are user-defined attribute of HTML element.

The attribute's name must start with data-.

A custom data attribute is an attribute in no namespace whose name starts with the string "data-", has at least one character after the hyphen, is XML-compatible, and contains no ASCII upper alphas.

[2020-04-20 from https://html.spec.whatwg.org/multipage/dom.html#custom-data-attribute]

Example of Custom Data Attribute

example of adding accessed date of a url as data attribute.

<a href="http://example.com/some" data-accessed="2025-03-07">http://example.com/some</a>

Using JavaScript to Access Custom Data Attribute

You can use JavaScript DOM getAttribute method, like any other attribute.

HTML custom tags and attribute