Add Policy Control section in the specification. (#112)

index.html

@@ -47,7 +47,12 @@
            href: 'https://w3c.github.io/device-posture/polyfill/demo.html'
          }]
         }],
-        xref: "web-platform",
+        xref: {
+          profile: "web-platform",
+          specs: [
+            "permissions-policy"
+          ]
+        },
         logos: [
         {
           src: "./images/daswg-logo.svg",
@@ -533,6 +538,41 @@ <h2>
         </div>
       </section>
     </section>
+    <section>
+      <h3>
+        Policy control
+      </h3>
+      <p>
+        The Device Posture API defines a [=policy-controlled feature=]
+        identified by the token "device-posture".
+        Its [=policy-controlled feature/default allowlist=] is `'self'`.
+      </p>
+      <aside class="note">
+        <p>
+          The [=policy-controlled feature/default allowlist=] of `'self'` allows usage in
+          same-origin nested frames but prevents third-party content from using
+          the feature.
+        </p>
+        <p>
+          Third-party usage can be selectively enabled by adding
+          `allow="device-posture"` attribute to the frame container element:
+        </p>
+        <pre class="example html" title=
+        "Enabling device posture on remote content">
+          &lt;iframe src="https://third-party.com" allow="device-posture"/&gt;&lt;/iframe&gt;
+        </pre>
+        <p>
+          Alternatively, the Device Posture API can be disabled completely by
+          specifying the permissions policy in a HTTP response header:
+        </p>
+        <pre class="example http" title="Feature Policy over HTTP">
+          Permissions-Policy: {"device-posture": []}
+        </pre>
+        <p>
+          See [[[PERMISSIONS-POLICY]]] for more details.
+        </p>
+      </aside>
+    </section>
     <section>
       <h2>
         Security and Privacy considerations