Bad value “1” for attribute “allowfullscreen” on element “iframe”.

The allowfullscreen attribute on an iframe element is a boolean attribute and should not be assigned a value like "1".

Boolean attributes in HTML don’t need a value. Their mere presence on an element means “true,” and their absence means “false.” When you write allowfullscreen="1", the W3C validator flags it because "1" is not a valid value for a boolean attribute.

According to the HTML specification, a boolean attribute can only have three valid forms: the attribute name alone (allowfullscreen), an empty string (allowfullscreen=""), or the attribute’s own name as the value (allowfullscreen="allowfullscreen"). Any other value, including "1", "true", or "yes", is technically invalid.

Invalid Example

<iframe
  src="https://www.example.com/video"
  allowfullscreen="1">
</iframe>

Valid Example

<iframe
  src="https://www.example.com/video"
  allowfullscreen>
</iframe>

Learn more:

MDN Reference

MDN: iframe element

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WHATWG Spec

WHATWG HTML Standard: iframe element

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