Use the following markup to turn the site into fluid flexbox layout with sticky footer. Credits for cross-browser solution go to Philip Walton.
Example
Header
Main Content
<body class="site">
<header role="banner" class="site-header">
<div class="kss__example-box">Header</div>
</header>
<main role="main" class="site-content">
<div class="kss__example-box">Main Content</div>
</main>
<footer role="contentinfo" class="site-footer">
<div class="kss__example-box">Sticky Footer</div>
</footer>
</body>