Improve sentence structure re TTML pruning

index.html

@@ -2446,7 +2446,7 @@ <h3>Validation warnings and errors</h3>
           </p>
           <p>The [[TTML2]] mechanism for dealing with vocabulary in unrecognised namespaces is
             to prune it prior to validation.
-            This approach can be used, however it could be reasonable for
+            This approach can be used; additionally it could be reasonable for
             an implementation to report as information those attributes and elements
             that have been pruned.
           </p>