From 13.2:The edges of the first page area establish the rectangle that is the initial containing block of the document.
From 10.5A percentage height on the root element is relative to the initial containing block.
This text must be displayed on an pale blue background.
When printed or displayed in a print preview, the background color must extend from one inch below the top of the page to one inch above the bottom of the page, and to within one inch of the sides. A thin blue border edges the background color.