Work around a Firefox computed style bug.

lib/src/runner/browser/static/host.css

Index: lib/src/runner/browser/static/host.css
diff --git a/lib/src/runner/browser/static/host.css b/lib/src/runner/browser/static/host.css
index f17cb8da116775655b5a33359d4e17b49bc26624..8cf273a8769a5fe30b91404f40aa261060a04a60 100644
--- a/lib/src/runner/browser/static/host.css
+++ b/lib/src/runner/browser/static/host.css
@@ -2,13 +2,15 @@
  * for details. All rights reserved. Use of this source code is governed by a
  * BSD-style license that can be found in the LICENSE file. */
 
-/* Compiled output from
- * http://codepen.io/mknadler/pen/11b75cb014a3c382f54abf527655af21. */
-
 iframe {
-  display: none;
+  /* We would use display: none here, but then Firefox fails to properly compute
+   * styles. See #274 */
+  visibility: hidden;
 }
 
+/* Compiled output from
+ * http://codepen.io/mknadler/pen/11b75cb014a3c382f54abf527655af21. */
+
 svg {
   position: absolute;
   margin: auto;