Floating div positioning

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I need a floating rectangle (100% width, 100px height) to appear exactly 20px above the bottom of a page. How can I do that?

The code below shows the rectangle at the bottom of the browser window not the page – so if the page is taller than what can fit in the screen, the rectangle appears somewhere in the middle of the page.

<div style="z-index: 1; position: absolute; right: 0px; bottom: 20px; background-color: #000