diff -u b/core/themes/bartik/css/components/tabs.css b/core/themes/bartik/css/components/tabs.css
--- b/core/themes/bartik/css/components/tabs.css
+++ b/core/themes/bartik/css/components/tabs.css
@@ -20,6 +20,8 @@
 }
 .tabs--primary .tabs__tab {
   display: block;
+  float: left;
+  vertical-align: bottom;
   margin: 0 5px 0 0; /* LTR */
 }
 [dir="rtl"] .tabs--primary .tabs__tab {
@@ -29,7 +31,7 @@
   color: #000;
   background-color: #ededed;
   border-color: #bbb;
-  border-style: solid solid none solid;
+  border-style: solid;
   border-width: 1px;
   height: 1.8em;
   line-height: 1.9;
@@ -45,7 +47,7 @@
 }
 
 @media screen and (min-width: 37.5em) { /* 600px */
-  .tabs--primary {
+  ul.tabs--primary {
     background: transparent url(../../images/tabs-border.png) repeat-x left bottom;
     padding: 0 3px;
   }
@@ -56,15 +58,18 @@
 }
 
 @media screen and (max-width: 37.5em) { /* 600px */
-  .tabs--primary {
+  ul.tabs--primary {
     border-bottom: 1px solid #bbb;
+    padding: 0;
   }
   .tabs--primary .tabs__tab {
     display: block;
+    float: none;
     margin: 0;
   }
   .tabs--primary .tabs__tab a {
     padding: 5px 10px;
+    border-style: solid solid none solid;
   }
   .tabs--primary .tabs__tab.is-active a {
     border-bottom: none;