From the help file:-

Normally if you have a long web page that can be scrolled in the browser, the browser background scrolls with it, so the page content remains in the same place relative to the background. But you can 'fix' the background so it doesn't scroll with the page content. Right-click on the pasteboard and select "Pasteboard background" > "Fixed". Then when you preview, the content slides over the fixed background as you scroll (to see this, make sure your page is long enough so the browser shows a scroll bar!).
To switch back to the default, scrolling, behavior, select "Pasteboard background" > "Scrolling".