diff --git a/index.html b/index.html
index 19b3f92..79f9d0c 100644
--- a/index.html
+++ b/index.html
@@ -7,6 +7,7 @@ html, body {
 }
 
 .checkerboard-bg {
+  /* https://codepen.io/pascalvgaal/pen/jPXPNP/ */
   background: #eee url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="400" height="400" fill-opacity=".1" ><rect x="200" width="200" height="200" /><rect y="200" width="200" height="200" /></svg>');
   background-size: 20px 20px;
 }