diff --git a/themes/jquery/js/main.js b/themes/jquery/js/main.js
index 48fa82fb..e1e51183 100644
--- a/themes/jquery/js/main.js
+++ b/themes/jquery/js/main.js
@@ -1,3 +1,6 @@
+// Some CSS depends on the `js` class.
+$( "html" ).addClass( "js" ).removeClass( "no-js" );
+
 /*
  * All sites
  */