Suddenly I saw that
body doesn't expand till the bottom of screen. There is
a gap bellow the footer.
Was
there some reason you were expecting otherwise?
The
document length is the document length. If it is shorter than the
browser window, it does not fill the browser window. This is normal.
Do
you want a fixed footer?
Do
you want a fixed footer, but only so long as the page is shorter
than the viewport?