Use correct syntax.
1 files changed, 2 insertions(+), 2 deletions(-)

M index.php
M index.php +2 -2
@@ 164,10 164,10 @@ 
       display: block;
       text-decoration: underline;
     }
-    th span.desc:after {
+    th span.desc::after {
       content: " ▼";
     }
-    th span.asc:after {
+    th span.asc::after {
       content: " ▲";
     }