diff --git a/src/css/dyna-rules.css b/src/css/dyna-rules.css
index b8c813aca..35bb5482b 100644
--- a/src/css/dyna-rules.css
+++ b/src/css/dyna-rules.css
@@ -18,7 +18,7 @@ code {
 }
 #diff > .pane {
     border: 0;
-    box-sizing: box-border;
+    box-sizing: border-box;
     display: inline-block;
     font: 12px/1.8 monospace;
     margin: 0;