diff --git a/core/themes/seven/style.css b/core/themes/seven/style.css
index f1cb442..92d7b31 100644
--- a/core/themes/seven/style.css
+++ b/core/themes/seven/style.css
@@ -735,6 +735,22 @@ details summary {
   padding-top: 0.5em;
   padding-bottom: 0.5em;
 }
+.js details .details-wrapper {
+  -webkit-transition: max-height 0.5s ease-in-out;
+  -moz-transition: max-height 0.5s ease-in-out;
+  -o-transition: max-height 0.5s ease-in-out;
+  transition: max-height 0.5s ease-in-out;
+}
+.js details:not([open]) .details-wrapper { /* typically this is 'display: none' */
+  max-height: 0;
+  overflow: hidden;
+  padding-bottom: 0;
+  padding-top: 0;
+  display: block;
+}
+.js details[open] .details-wrapper {
+  max-height: 100000px; /* something 'impossibly' large */
+}
 
 /**
  * Form elements.