Added a new issue box.

index.html

@@ -1400,6 +1400,14 @@ <h3>
           URL</var>. The output from inputting an JSON document into this
           algorithm is a <dfn>processed manifest</dfn>.
         </p>
+        <p class="issue">
+          There is currently no way to invoke this algorithm without a
+          <var>document URL</var>. However, several user agents do just that
+          (when they install a web application directly from a manifest URL,
+          without a document). The algorithms that invoke this one should
+          similarly be written to not require a <var>document URL</var>. See
+          <a href="https://github.com/w3c/manifest/issues/668">#668</a>.
+        </p>
         <p class="issue">
           We need to catch throws associated with enumerations in IDL
           conversion as the spec might gain new values over time not supported