fix: use window.onload

src/index.js

@@ -188,7 +188,7 @@ document.getElementById("user-input").addEventListener("keydown", (event) => {
   }
 });
 
-document.onload = function () {
+window.onload = function () {
   document.getElementById("download").textContent = "Download";
   document.getElementById("download").disabled = false;
 }