validator did not like this:

Element form not allowed as child of element span in this
context. (Suppressing further errors from this subtree.)

So, I moved the id into the form rather than a span.

www/conservancy/templates/frontpage.html

@@ -49,15 +49,12 @@
       and other social networks, too.  Look
       for us everywhere and follow/like us!</p>
 
-    <span id="subbox">
-        <form method="post" action="https://lists.sfconservancy.org/mailman/subscribe/announce">
+        <form id="subbox" method="post" action="https://lists.sfconservancy.org/mailman/subscribe/announce">
           You can also subscribe to Conservancy's low-traffic email announce
           list
           (and <a href="http://lists.sfconservancy.org/pipermail/announce/">read
           its archives</a>): <br/><input name="email" placeholder="person@example.org"/><input type="submit" name="email-button" value="Subscribe"/>
         </form>
-      </span>
-
 </div>
 
 <div class="column column-large">