Extract encoding properly in fetch a classic script

Builds upon whatwg/fetch#1447.

Author: annevk

source

@@ -2526,6 +2526,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://fetch.spec.whatwg.org/#http-cors-protocol">CORS protocol</dfn></li>
      <li><dfn data-x="default-user-agent-value" data-x-href="https://fetch.spec.whatwg.org/#default-user-agent-value">default `<code>User-Agent</code>` value</dfn></li>
      <li><dfn data-x-href="https://fetch.spec.whatwg.org/#concept-header-extract-mime-type">extract a MIME type</dfn></li>
+     <li><dfn data-x-href="https://fetch.spec.whatwg.org/#legacy-extract-an-encoding">legacy extract an encoding</dfn></li>
      <li><dfn data-x="concept-fetch" data-x-href="https://fetch.spec.whatwg.org/#concept-fetch">fetch</dfn></li>
      <li><dfn data-x-href="https://fetch.spec.whatwg.org/#concept-http-redirect-fetch">HTTP-redirect fetch</dfn></li>
      <li><dfn data-x-href="https://fetch.spec.whatwg.org/#ok-status">ok status</dfn></li>
@@ -93384,9 +93385,17 @@ document.querySelector("button").addEventListener("click", bound);
     unlike the other script-fetching algorithms in this section.</p>
    </li>
 
-   <li><p>If <var>response</var>'s <span data-x="Content-Type">Content Type metadata</span>, if
-   any, specifies a character encoding, and the user agent supports that encoding, then set
-   <var>character encoding</var> to that encoding (ignoring the passed-in value).</p></li>
+   <li><p>Let <var>potentialMIMETypeForEncoding</var> be the result of <span data-x="extract a MIME
+   type">extracting a MIME type</span> given <var>response</var>'s <span
+   data-x="concept-response-header-list">header list</span>.</p></li>
+
+   <li>
+    <p>Set <var>character encoding</var> be the result of <span data-x="legacy extract an
+    encoding">legacy extracting an encoding</span> given <var>potentialMIMETypeForEncoding</var> and
+    <var>character encoding</var>.</p>
+
+    <p class="note">This intentionally ignores the <span>MIME type essence</span>.</p>
+   </li>
 
    <li>
     <p>Let <var>source text</var> be the result of <span data-x="decode">decoding</span>