What is the opposite of :hover (on mouse leave)?

Question

If I understand correctly you could do the same thing by moving your transitions to the link rather than the hover state:

ul li a {
    color:#999;       
    transition: color 0.5s linear; /* vendorless fallback */
    -o-transition: color 0.5s linear; /* opera */
    -ms-transition: color 0.5s linear; /* IE 10 */
    -moz-transition: color 0.5s linear; /* Firefox */
    -webkit-transition: color 0.5s linear; /*safari and chrome */
}

ul li a:hover {
    color:black;
    cursor: pointer;
}

http://jsfiddle.net/spacebeers/sELKu/3/

The definition of hover is:

The :hover selector is used to select elements when you mouse over them.

By that definition the opposite of hover is any point at which the mouse is not over it. Someone far smarter than me has done this article, setting different transitions on both states - http://css-tricks.com/different-transitions-for-hover-on-hover-off/

#thing {
   padding: 10px;
   border-radius: 5px;

  /* HOVER OFF */
   -webkit-transition: padding 2s;
}

#thing:hover {
   padding: 20px;
   border-radius: 15px;

  /* HOVER ON */
   -webkit-transition: border-radius 2s;
}

Just use CSS transitions instead of animations.

A {
    color: #999;
    transition: color 1s ease-in-out;
}

A:hover {
    color: #000;
}

Live demo

The opposite is using :not

e.g.

selection:not(:hover) { rules }

No there is no explicit property for mouse leave in CSS.

You could use :hover on all the other elements except the item in question to achieve this effect. But Im not sure how practical that would be.

I think you have to look at a JS / jQuery solution.