include a default transition for body opacity (which we never clean up)

src/unresolved.js

@@ -4,18 +4,23 @@
  * license that can be found in the LICENSE file.
  */
 (function(scope) {
-  
+
   // TODO(sorvell): It's desireable to provide a default stylesheet 
   // that's convenient for styling unresolved elements, but
   // it's cumbersome to have to include this manually in every page.
   // It would make sense to put inside some HTMLImport but 
   // the HTMLImports polyfill does not allow loading of stylesheets 
   // that block rendering. Therefore this injection is tolerated here.
-  var UNRESOLVED = 'unresolved';
-  var UNRESOLVED_SELECTOR = 'body[' + UNRESOLVED + ']';
+
   var style = document.createElement('style');
-  style.textContent = UNRESOLVED_SELECTOR + ' { ' +
-      'opacity: 0; display: block; overflow: hidden; } \n';
+  style.textContent = ''
+      + 'body {'
+      + 'transition: opacity ease-in 0.2s;' 
+      + ' } \n'
+      + 'body[unresolved] {'
+      + 'opacity: 0; display: block; overflow: hidden;' 
+      + ' } \n'
+      ;
   var head = document.querySelector('head');
   head.insertBefore(style, head.firstChild);