so you want the footer to disappear off the page if the viewer scrolls up.
Just add a section or a div as the last element on the page.
You don't have to align the div to bottom if it's the last section (div).
And divs are automatically 100% width if displaytype is block.
If you use a section (in case you don't know: a wf section is not the same thing as an html5 section)...
you can add a container element (which is basically a smart div) in the section
so that all contained elements within the container are responsive to the device.
If you don't add a container to the section or just use a div instead of a section
the you will need to handle the responsiveness
If you were to do this in bootstrap or something else...
you would create a div (section) and place another div (container) inside it
that would sized and auto-margined to your liking.