Bad value “rocketlazyloadscript” for attribute “type” on element “script”: Subtype missing.

Understanding the Issue

MIME types follow a specific format: a type and a subtype separated by a forward slash, such as text/javascript or application/json. When the W3C validator encounters type="rocketlazyloadscript" on a <script> element, it flags it because this value has no slash and no subtype — it doesn’t conform to the MIME type syntax defined in the HTML specification.

The value rocketlazyloadscript is intentionally set by the WP Rocket WordPress caching and performance plugin. WP Rocket changes the type attribute of <script> elements from text/javascript (or removes the default) and replaces it with this custom value. This prevents the browser from executing the script immediately on page load. WP Rocket’s JavaScript then swaps the type back to a valid value when the script should actually be loaded, achieving a lazy-loading effect that can improve page performance.

Why This Is Flagged

The HTML specification states that if the type attribute is present on a <script> element, its value must be one of the following:

• A valid JavaScript MIME type (e.g., text/javascript) — indicating the script should be executed.
• The string module — indicating the script is a JavaScript module.
• Any other valid MIME type that is not a JavaScript MIME type — indicating a data block that the browser should not execute.

Since rocketlazyloadscript is not a valid MIME type at all (it lacks the type/subtype structure), it fails validation. While browsers will simply ignore scripts with unrecognized type values (which is exactly what WP Rocket relies on), the markup itself is technically non-conforming.

How to Fix It

Because this value is generated by a plugin rather than authored manually, your options are:

1. Configure WP Rocket to exclude specific scripts — In WP Rocket’s settings under “File Optimization,” you can exclude individual scripts from the “Delay JavaScript execution” feature. This prevents WP Rocket from modifying their type attribute.

2. Disable delayed JavaScript execution entirely — If W3C compliance is critical, you can turn off the “Delay JavaScript execution” option in WP Rocket. This eliminates the validation errors but sacrifices the performance benefit.

3. Accept the validation errors — WP Rocket acknowledges these errors in their documentation and considers them an expected side effect of their optimization technique. Since browsers handle the non-standard type gracefully (by not executing the script), there is no functional or accessibility issue. The errors are purely a standards compliance concern.

Examples

Invalid: Non-standard type value (generated by WP Rocket)

<script type="rocketlazyloadscript" src="/js/analytics.js"></script>

The validator reports: Bad value “rocketlazyloadscript” for attribute “type” on element “script”: Subtype missing.

Valid: Standard type attribute

<script type="text/javascript" src="/js/analytics.js"></script>

Valid: Omitting the type attribute entirely

Since text/javascript is the default for <script> elements in HTML5, the type attribute can be omitted entirely:

<script src="/js/analytics.js"></script>

Valid: Using a module script

<script type="module" src="/js/app.js"></script>

Valid: Using a data block with a proper MIME type

If you need a non-executed data block, use a valid MIME type that isn’t a JavaScript type:

<script type="application/json" id="config">
  {"lazy": true, "threshold": 200}
</script>

Learn more:

docs.wp-rocket.me

WP Rocket: W3C Validation errors

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WHATWG Spec

HTML Spec: Script type

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MDN Reference

MDN: Script element

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