How you can keep the footer
below the page always, but without using fixed or absolute, because you can give several cases:
In the event that the content is not enough to fill the page, the footer does not stay down unless you put a position: fixed; bottom: 0
or position: absolute; bottom: 0
But if I put one of those two, since the content is bigger than the screen, the footer is always visible, in the case of fixed it scrolls but it always remains on the screen, but in the case of setting absolute, the going down stays in the middle of the content.
<html>
<body>
<header style="background:grey">Header</header>
<div style="background:yellow">Contenido</div>
<footer style="background:green">tiene que estar abajo pero no se queda abajo siempre...</footer>
</body>
</html>