tweak 1) dialog was visible before first opening on scroll.

tweak 2) dialog was removed, not just closed.

script.js

@@ -134,7 +134,7 @@ function openUsageDialog() {
   const dialog = document.querySelector('#usage');
   dialog.showModal();
   dialog.addEventListener('click', () => {
-    dialog.remove();
+    dialog.close();
   });
 }
 

styles.css

@@ -157,7 +157,7 @@ h2 img {
     box-shadow: inset 0 0 1em #F0F7;
 }
 
-dialog {
+dialog:modal {
     margin: auto;
     padding: 2em;
     display: flex;
@@ -203,4 +203,4 @@ dialog button {
     border: thin dashed #000;
     border-radius: 0.2em;
     font-weight: bold;
-}
+}