Update normative requirement on securing verifiable credentials.

index.html

@@ -645,10 +645,10 @@ <h2>Core Data Model</h2>
 this specification. However, the concepts of a <a>verifiable credential</a> or a
 <a>verifiable presentation</a> are defined as <a>conforming documents</a> and do
 have associated media types. The concrete difference between these concepts
-&mdash; between <a>credential</a> and <a>presentation</a> Vs. <a>verifiable
+&mdash; between <a>credential</a> and <a>presentation</a> vs. <a>verifiable
 credential</a> and <a>verifiable presentation</a> &mdash; is simply the fact
 that the "verifiable" objects are secured in a way that is cryptographically
-verifiable, which the others are not. For more details, see Section
+verifiable, and the others are not. For more details, see Section
 <a href="#securing-verifiable-credentials"></a>.
       </p>
 
@@ -1813,9 +1813,9 @@ <h4>Representing Time</h4>
         <h3>Securing Verifiable Credentials</h3>
 
         <p>
-At least one securing mechanism, and the details necessary to evaluate it, MUST
-be expressed to construct a <a>verifiable credential</a> or a
-<a>verifiable presentation</a>.
+A securing mechanism MUST express the details necessary to evaluate whether a
+<a>verifiable credential</a> or <a>verifiable presentation</a> has been secured
+using it.
         </p>
         <p>
 This specification recognizes two classes of securing mechanisms: those that use