Bad value “” for attribute “is” on element “a”: Must be non-empty.

The is attribute on the <a> element is present but set to an empty string. This attribute names a customized built-in element, so it must hold a valid custom element name and cannot be empty.

The is attribute tells the browser to upgrade a standard element into a customized built-in element. You register one in JavaScript with customElements.define("fancy-link", FancyLink, { extends: "a" }), then write <a is="fancy-link"> to apply it. The value has to match a registered custom element name, which is always non-empty and contains a hyphen. An empty is="" names nothing, so the validator rejects it.

In practice this usually comes from a template that prints is="" when the variable feeding it is blank. If the element is not meant to be a customized built-in, remove the is attribute entirely. If it is, set the name of the element you registered.

Note that Safari does not support customized built-in elements, so many projects avoid is and use autonomous custom elements (a hyphenated tag such as <fancy-link>) instead.

Invalid example

<a is="" href="/pricing">Pricing</a>

Valid example

Remove is when the link is an ordinary anchor:

<a href="/pricing">Pricing</a>

Or give it the name of a registered customized built-in element:

<a is="fancy-link" href="/pricing">Pricing</a>

Learn more:

MDN Reference

MDN - the is attribute

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MDN Reference

MDN - Using custom elements

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WHATWG Spec

HTML Standard - Custom elements

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