Need help with a Mega Menu on a Drupal Site. The dropdown is being shown using the following css....

Code:
#primary ul li:hover ul {
display: block;
}
#primary ul li ul {
position: absolute;
display: none;
top: 42px;
left: -21px;
margin: 0;
padding: 0 0 20px 0;
width: 960px;
height: auto;
background: #071689;
border: 0;
line-height: normal;
overflow: hidden;
}
Now I know that with display changing I cannot use CSS3 Transitions correct? So I need to find an alternative way to make the mega menu "Fade-In" rather than instantly pop-up. I could always resort to JQuery I suppose but trying to stay with CSS. Thanks in advance for your replies! =)