transition

div {
  transition: all 0.5s ease;
  background: red;
  padding: 10px;
}
div:hover {
  background: green;
  padding: 20px;
}

#content #nav a:hover {
    color: black;
    background-color: #AD310B;
    /* Firefox */
    -moz-transition: all 1s ease-in;
    /* WebKit */
    -webkit-transition: all 1s ease-in;
    /* Opera */
    -o-transition: all 1s ease-in;
    /* Standard */
    transition: all 1s ease-in;
}