A “script” element with a “type” attribute whose value is neither a JavaScript MIME type, “module”, “importmap”, nor “speculationrules” (i.e., a data block) must not have a “defer” attribute.

The defer attribute is invalid on a <script> element whose type marks it as a data block, because browsers never execute data blocks.

When the type attribute is anything other than a JavaScript MIME type, module, importmap, or speculationrules, the browser treats the element as a data block: inline content that other scripts on the page can read, but that is never fetched or run. The most common example is application/ld+json structured data. Since nothing executes, there is nothing to defer, and the attribute is invalid.

This usually happens when a template or a performance plugin adds defer to every <script> tag on the page, including the data blocks.

Examples

Invalid: defer on a data block

<script type="application/ld+json" defer>
  {
    "@context": "https://schema.org",
    "@type": "Organization",
    "name": "Example"
  }
</script>

Valid: data block without defer

<script type="application/ld+json">
  {
    "@context": "https://schema.org",
    "@type": "Organization",
    "name": "Example"
  }
</script>

Remove the defer attribute. If the script was actually meant to run, fix the type value instead: use a JavaScript MIME type, module, or omit the attribute entirely.

Learn more:

MDN Reference

HTML script element - MDN

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MDN Reference

The script type attribute - MDN

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WHATWG Spec

Scripting - WHATWG HTML Standard

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