Defines how the background image will behave when scrolling the page.
default
background-attachment: scroll;
The background image will scroll with the page. It will also position and resize itself according to the element it's applied to.
background-attachment: fixed;
The background image will not scroll with the page, and remain positioned according to the viewport. It will also position and resize itself according to the viewport. As a result, the background image will probably only be partially visible.