Suppose you do nothing to manipulate the HTML document’s default layout. In that case, browsers will display the above snippet in its normal flow like so:
Suppose you used absolute CSS positioning to manipulate the default layout of the “Green” list item. In that case, browsers will display the HTML document out of flow like so:
In other words, a normal flow CSS layout makes browsers display a page’s elements precisely as you arranged them in the source code. But you take a document out of flow once you alter its default layout.