One of the recent popular trends in web design has been a vertical parallax scroll effect that moves the background of a page at a different speed to the actual content. The intention is to create the illusion of depth within the page by making the background appear further away.
Yep, that will do nicely.
OK, let's fix this image to the top of our document so that it initially doesn't scroll with the page:
(There now follows some dummy text to ensure the page scrolls on even the largest monitors...)
Tested on desktop versions of Chrome, IE11, Edge, Safari, Opera and Firefox. Any problems are entirely your own.