8.1.3.2 Fetching scripts says:

• Under "To fetch a single module script", step 8: "If any of the following conditions are met [...] The result of extracting a MIME type from response's header list (ignoring parameters) is not a JavaScript MIME type."
• There are no equivalent rules for classic or worker scripts.

Chrome would like to be more strict about the non-module scripts, too. On Chrome's beta channel, we see:

• ca. 0.01% of page loads contain worker scripts (workers or scripts loaded from workers) that would fail this check if it were applied.
• ca. 6% of classic, non-worker page loads contain scripts that would fail this check if applied
  • of these, the vast majority ( ~3/4 ) are text/html
  • ~1/4 text/plain
  • ~1/10 application/octet-stream
  • the rest is noise, <0.01%

These numbers would probably support blocking non-script MIME types for the "fetch a classic worker script" and "fetch a classic worker-imported script" cases, too, but not (yet) for all script types.

Would this make sense?

@mikewest