[w3c/ServiceWorker] importScript MIME type checking failure (#1288)

The service worker spec currently specifies that we should override the behavior for [importScripts()](https://w3c.github.io/ServiceWorker/#importscripts): 
_Extract a MIME type from the response’s header list. If this MIME type (ignoring parameters) is not a JavaScript MIME type, return a network error._

Note that in the HTML spec, [importScript](https://html.spec.whatwg.org/multipage/workers.html#dom-workerglobalscope-importscripts
) does the following:
1. _Fetch a classic worker-imported script given url and settings object, passing along any custom perform the fetch steps provided. If this succeeds, let script be the result. Otherwise, rethrow the exception._
2. _Run the classic script script, with the rethrow errors argument set to true._

In a [different section](https://html.spec.whatwg.org/multipage/webappapis.html#fetch-a-single-module-script) of the spec where it does say to check it, it calls out the scenario where it doesn’t:
_For historical reasons, fetching a classic script does not include MIME type checking. In contrast, module scripts will fail to load if they are not of a correct MIME type._

Considering that all browser are not implementing the service worker spec text override, should we remove it from the spec?

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https://github.com/w3c/ServiceWorker/issues/1288