URL being validated or code to reproduce error:

<script type="speculationrules">

But the validator shows:

Error: Bad value speculationrules for attribute type on element script: Subtype missing.

See

• https://developer.mozilla.org/en-US/docs/Web/API/Speculation_Rules_API#concepts_and_usage
• https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/script/type/speculationrules
• https://developer.chrome.com/blog/speculation-rules-improvements
• https://developer.chrome.com/docs/web-platform/prerender-pages

I suspect the real issue is that Chrome have chosen a type which isn't a MIME type as per https://html.spec.whatwg.org/multipage/scripting.html#the-script-element