how do I keep my footer at the bottom of the page with css? code example

Example 1: why is my footer not at the bottom html

#footer {
   position:fixed;
   left:0px;
   bottom:0px;
   height:30px;
   width:100%;
   background:#999;
}

//name footer id="footer"

Example 2: keep footer at bottom of page

#footer {
  position: relative;
  margin-top: -180px;
  /* negative value of footer height */
  height: 180px;
  clear: both;
 /*Custom styling*/
}

/* Opera Fix thanks to Maleika (Kohoutec) */

body:before {
  content: "";
  height: 100%;
  float: left;
  width: 0;
  margin-top: -32767px;
  /* thank you Erik J - negate effect of float*/
}