For reference, please see this website
I want to use the faux columns method
to ensure the blue background of the #secondary sidebar extends to the bottom of its parent: #main.
However, when I use Firefox Developer Tools Responsive View, and reduce the width of the viewport, #secondary's width becomes a %, but #main's background image remains fixed, so the blue part of the background image bleeds into the #primary content area.
I once read an article where someone used the faux columns method and was able to set the position of the background image to scale down when the viewport was scaled down, so that the colour change of the background image always lined up with the right hand side of the #secondary sidebar.
I am aware you can set the background-position to a %, but this matches a specific point x% in from the left of the image with a point x% in from the left of its parent, so it always stays in the same position regardless of what % you specify.