Transitions on the CSS display property

I'm currently designing a CSS 'mega dropdown' menu - basically a regular CSS-only dropdown menu, but one that contains different types of content.

At the moment, it appears that CSS 3 transitions don't apply to the 'display' property, i.e., you can't do any sort of transition from display: none to display: block (or any combination).

Is there a way for the second-tier menu from the above example to 'fade in' when someone hovers over one of the top level menu items?

I'm aware that you can use transitions on the visibility: property, but I can't think of a way to use that effectively.

I've also tried using height, but that just failed miserably.

I'm also aware that it's trivial to achieve this using JavaScript, but I wanted to challenge myself to use just CSS, and I think I'm coming up a little short.

Solution

You can concatenate two transitions or more, and visibility is what comes handy this time.

div {
  border: 1px solid #eee;
}
div > ul {
  visibility: hidden;
  opacity: 0;
  transition: visibility 0s, opacity 0.5s linear;
}
div:hover > ul {
  visibility: visible;
  opacity: 1;
}

<div>
  <ul>
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
  </ul>
</div>

(Don't forget the vendor prefixes to the transition property.)