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

The allowfullscreen attribute on an <iframe> is a boolean attribute and does not accept a value like "yes".

Boolean attributes in HTML work by their presence or absence alone. When a boolean attribute is present on an element, it means "true." When it is absent, it means "false." Valid ways to write a boolean attribute are: the attribute name with no value, an empty string value (""), or the attribute name itself as the value. Assigning "yes", "true", or any other string is invalid.

This applies to all boolean attributes in HTML, such as disabled, checked, autoplay, muted, and allowfullscreen.

Invalid example

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

Valid example

Any of these three forms is valid:

<!-- Attribute name only (most common) -->
<iframe
  src="https://example.com/video"
  allowfullscreen>
</iframe>

<!-- Empty string value -->
<iframe
  src="https://example.com/video"
  allowfullscreen="">
</iframe>

<!-- Attribute name as the value -->
<iframe
  src="https://example.com/video"
  allowfullscreen="allowfullscreen">
</iframe>

The first form, with just allowfullscreen and no value, is the most widely used and the most readable.

Learn more:

MDN Reference

HTML iframe element - allowfullscreen attribute

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WHATWG Spec

WHATWG HTML Standard - iframe element

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