diff -u b/core/themes/seven/style.css b/core/themes/seven/style.css
--- b/core/themes/seven/style.css
+++ b/core/themes/seven/style.css
@@ -497,7 +497,7 @@
 
 .is-horizontal .tabs.secondary .tabs__tab {
   background: none;
-  border-width: 1px 1px 2px;
+  border-width: 0 0 2px;
   float: left;
   position: relative;
   top: -1px;