diff --git a/libreplan-webapp/src/main/webapp/planner/css/ganttzk.css b/libreplan-webapp/src/main/webapp/planner/css/ganttzk.css
index b27383b2f..d298dd6ac 100644
--- a/libreplan-webapp/src/main/webapp/planner/css/ganttzk.css
+++ b/libreplan-webapp/src/main/webapp/planner/css/ganttzk.css
@@ -416,6 +416,7 @@ div.box.limiting-unassigned {
 
 #timetrackerheader .second_level_ td {
     -moz-box-sizing: border-box;
+    box-sizing: border-box;
     border-right: solid 1px #9EB6CE;
 }