Enforce strict MIME type checks for worker-imported scripts

This addresses a portion of the proposal in whatwg#3255.

source

@@ -86975,10 +86975,23 @@ interface <dfn>ApplicationCache</dfn> : <span>EventTarget</span> {
 
    <li><p>Let <var>response</var> be <var>response</var>'s <span>unsafe response</span>.</p></li>
 
-   <li><p>If <var>response</var>'s <span data-x="concept-response-type">type</span> is "<code
-   data-x="">error</code>", or <var>response</var>'s <span
-   data-x="concept-response-status">status</span> is not an <span>ok status</span>, throw a
-   <span>"<code>NetworkError</code>"</span> <code>DOMException</code>.</p></li>
+   <li>
+    <p>If any of the following conditions are met, throw a <span>"<code>NetworkError</code>"</span>
+    <code>DOMException</code>:</p>
+
+    <ul class="compact">
+     <li><p><var>response</var>'s <span data-x="concept-response-type">type</span> is "<code
+     data-x="">error</code>"</p></li>
+
+     <li><p><var>response</var>'s <span data-x="concept-response-status">status</span> is not an
+     <span>ok status</span></p></li>
+
+     <li>
+      <p>The result of <span data-x="extract a MIME type">extracting a MIME type</span> from
+      <var>response</var>'s <span data-x="concept-response-header-list">header list</span> is not a
+      <span>JavaScript MIME type</span></p>
+    </ul>
+   </li>
 
    <li><p>Let <var>source text</var> be the result of <span data-x="UTF-8 decode">UTF-8
    decoding</span> <var>response</var>'s <span data-x="concept-response-body">body</span>.</p></li>