Here is a solution that does not require that the footer be placed outside of the main wrapper element, which is how most people structure their pages.
html,
body {
margin: 0;
height: 100%;
}
.wrapper {
box-sizing: border-box;
position: relative;
padding-bottom: 1em; /* Height of footer */
min-height: 100%;
}
header {
background-color: #cff;
}
footer {
position: absolute;
bottom: 0;
width: 100%;
color: #fff;
background-color: #000;
}
<div class="wrapper">
<header>I am the header.</header>
<article>I am content that doesn't fill the page. The footer will appear at the bottom of the browser window. However, when I do fill the page, you will need to scroll down to see the footer.</article>
<footer>I am the footer.</footer>
</div>
Explanation
The wrapper element will fill 100% of the viewport height. (You could also use 100vh for the wrapper if you don't want to set the height of the html and body elements.) The wrapper also has a bottom padding to create a placeholder for the footer to sit.
The footer is absolutely positioned to the bottom of the wrapper and sits in the placeholder created by the wrapper's bottom padding.
This means that when the page does not have scrollbars, the footer will be positioned at the very bottom. However, when there is enough content for scrollbars to appear, the footer will be pushed down below the content.
(The color
and background-color
CSS properties in the example are for decoration only, obviously. They are included so that when you run the code, you can clearly see the separated sections.)