Start the download, then feel free to reload the page or close the tab entirely. The download will continue in the background while you're away. When you return to the page, the download progress will automatically resume displaying (though it may take a second or two for updates to appear). You can cancel the ongoing download anytime.
On mobile, you can even kill the browser and the download will keep going (progress is displayed as a silent notification). Silent notifications are shown in an expandable section when you swipe down from the top of your screen. Look for the icon.
When the download succeeds, the file will be cached using the Cache API and be available offline.
Your browser doesn't support the Background Fetch API.