Attribute

An attribute extends an element, changing its behavior or providing metadata.

An attribute always has the form name="value" (the attribute's identifier followed by its associated value).

You may see attributes without the equals sign or a value. That is a shorthand for providing the empty string in HTML, or the attribute’s name in XML.

<input required>
<!-- is the same as… -->
<input required="">
<!-- or -->
<input required="required">

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Technical reference