WIP: Follow the form recommended by the Infra Standard for the declar…

…ation of algorithms

index.bs

@@ -875,15 +875,18 @@ spec: encrypted-media-draft; for: EME; urlPrefix: https://w3c.github.io/encrypte
         </p>
       </section>
 
-      <section>
-        <h4 algorithm>
-          Check a MIME Type
+      <section algorithm>
+        <h4 id="check-mime-type">
+          <dfn abstract-op noexport lt="Check a MIME Type|check a MIME Type">
+            Check a MIME Type
+          </dfn>
         </h4>
         <p>
-          The <dfn id="check-a-mime-type">check a MIME Type</dfn> algorithm takes a {{DOMString}} |contentType|, string |media|, and either
-          a {{MediaDecodingType}} or {{MediaEncodingType}} |encodingOrDecodingType|. It
-          returns either failure, or a boolean that represents if the MIME type of |contentType| is [=supported by the user agent=].
-          The following steps are run:
+          To [$check a MIME Type$], given a {{DOMString}} |contentType|, a
+          string |media|, and either a {{MediaEncodingType}} or
+          {{MediaDecodingType}} |encodingOrDecodingType|, perform the following
+          steps. They return either failure or a boolean that indicates whether
+          the MIME type of |contentType| is [=supported by the user agent=]:
         </p>
         <ol>
           <li>