no need for a dedicated class just to hide it

src/platform/web/dom/download.js

@@ -20,7 +20,7 @@ export async function downloadInIframe(container, iframeSrc, blob, filename) {
         iframe = document.createElement("iframe");
         iframe.setAttribute("sandbox", "allow-scripts allow-downloads allow-downloads-without-user-activation");
         iframe.setAttribute("src", iframeSrc);
-        iframe.className = "downloadSandbox";
+        iframe.className = "hidden";
         container.appendChild(iframe);
         let detach;
         await new Promise((resolve, reject) => {

src/platform/web/ui/css/main.css

@@ -49,7 +49,3 @@ body.hydrogen {
 input::-ms-clear {
     display: none;
 }
-
-.hydrogen > iframe.downloadSandbox {
-    display: none;
-}