Add algorithm to extract an encoding from a MIME type

This is useful for HTML's fetch a classic script and Fetch's upcoming opaque-response blocking. It will also be useful to more tightly define equivalent legacy setups elsewhere.

Author: annevk

fetch.bs

@@ -3491,6 +3491,30 @@ Content-Type:
  </table>
 </div>
 
+<p>To <dfn export>legacy extract an encoding</dfn> given failure or a <a for=/>MIME type</a>
+<var>mimeType</var> and an <a for=/>encoding</a> <var>fallbackEncoding</var>, run these steps:
+
+<ol>
+ <li><p>If <var>mimeType</var> is failure, then return <var>fallbackEncoding</var>.
+
+ <li><p>If <var>mimeType</var>["<code>charset</code>"] does not <a for=map>exist</a>, then return
+ <var>fallbackEncoding</var>.
+
+ <li><p>Let <var>tentativeEncoding</var> be the result of <a for=/>getting an encoding</a> from
+ <var>mimeType</var>["<code>charset</code>"].
+
+ <li><p>If <var>tentativeEncoding</var> is failure, then return <var>fallbackEncoding</var>.
+
+ <li><p>Return <var>tentativeEncoding</var>.
+</ol>
+
+<div class=note>
+ <p>This algorithm allows <var>mimeType</var> to be failure so it can be more easily combined with
+ <a>extract a MIME type</a>.
+
+ <p>It is denoted as legacy as modern formats are to exclusively use <a for=/>UTF-8</a>.
+</div>
+
 
 <h3 id=x-content-type-options-header>`<code>X-Content-Type-Options</code>` header</h3>