The transparent part should be pretty straightforward. Make the bottom layer transparent and make the html div element the canvas is on transparent as well. You should be able to the have that div overlay the rest of the website. Anyways, that’s the rough idea and can give some points to google.
Having input events pass through the canvas to the website below is another matter. Be default input is absorbed but there may be a way to let it pass through with JavaScript. Worst case you’ll have to manually remove event handlers the construct has by default.