Div fixed at bottom

HTML

<div id="longdiv">
Longdiv is long...
</div>
<div id="footer">
<p>Blah</p>
</div>

CSS

html, body {
  margin: 0;
  padding:0;
}

#longdiv {
  height: 2000px;
}
#footer {
  position: fixed;
  background: grey;
  width: 100%;
  bottom: 0;
}