渐变的CSS3动画[重复]

nav ul#menu li a {
    display: block;
    position: relative;
    z-index: 1;

    /* webkit gradient background */
    background: -webkit-linear-gradient(top, rgb(204, 0, 51), rgb(153, 0, 0));
}

nav ul#menu li a:before {
    width: 100%;
    height: 100%;
    display: block; 
    position: absolute;
    top: 0;
    left: 0;
    z-index: -1;
    opacity: 0;

    /* webkit gradient background */
    background: -webkit-linear-gradient(top, rgb(0, 156, 204), rgb(0, 111, 145));

    /* webkit transition */
    -webkit-transition: all 250ms linear;

    /* before hack */
    content: \".\";
    text-indent: -99999px;
}

nav ul#menu li a:hover:before {
    opacity: 1;
}

<div></div>

div {
    width: 200px;
    height: 200px;
    position: relative;
}

div::before, div::after {
    display: block;
    content: \'\';
    position: absolute;
    top: 0; right: 0; bottom: 0; left: 0;
}

.button {
    background: -webkit-gradient(linear, left top, right bottom, color-stop(0%,#fe046e), color-stop(100%,#a02ed4));
    -webkit-background-clip: text;
    color: rgba(255,255,255,1);

    -webkit-transition: all .2s ease-in-out;
            transition: all .2s ease-in-out;
}

.button:hover {
    background: -webkit-gradient(linear, left top, right bottom, color-stop(0%,#fe046e), color-stop(100%,#a02ed4));
    -webkit-background-clip: text;
    color: rgba(255,255,255,0);
}