When a document contains:

  <link rel="preload" href="main.js" as="worker" />

Safari reports:

  <link rel=preload> must have a valid `as` value


Probably a couple of tests are missing from 
https://wpt.fyi/results/html/syntax/speculative-parsing/generated/page-load?label=master&label=experimental&aligned&view=subtest&q=preload

or https://wpt.fyi/results/fetch/metadata/preload.https.sub.html?label=master&label=experimental&aligned&view=subtest&q=preload

or in 
https://wpt.fyi/results/preload/reflected-as-value.html?label=master&label=experimental&aligned&view=subtest&q=preload



The fetch spec definition:
https://fetch.spec.whatwg.org/#concept-request-destination

> A request has an associated destination, which is the empty string, "audio", "audioworklet", "document", "embed", "font", "frame", "iframe", "image", "manifest", "object", "paintworklet", "report", "script", "serviceworker", "sharedworker", "style", "track", "video", "worker", or "xslt". Unless stated otherwise it is the empty string.
> 
> A request’s destination is script-like if it is "audioworklet", "paintworklet", "script", "serviceworker", "sharedworker", or "worker".


It seems there are a couple of values which are not defined such as video for example.