diff --git a/demo/style.css b/demo/style.css
index 70ef8ed1..7b4ac814 100644
--- a/demo/style.css
+++ b/demo/style.css
@@ -100,4 +100,5 @@ pre {
 }
 #texture-atlas canvas {
     image-rendering: pixelated;
+    border: 1px solid #ccc;
 }