问伪元素上的转换:after和: working working

EN

:hover {
    text-decoration: none;
    -webkit-transition: all 0.2s ease-in-out;
       -moz-transition: all 0.2s ease-in-out;
        -ms-transition: all 0.2s ease-in-out;
         -o-transition: all 0.2s ease-in-out;
            transition: all 0.2s ease-in-out;
}
.play-icon:hover:after {
    position:absolute;
    cursor: pointer;
    top:0;
    left:0;
    content:'';
    width: 100%;
    height: 100%;
    background: url("http://www.kiddyjunction.ca/support/images/ilightbox/light-skin/thumb-overlay-play.png");
    background-repeat: no-repeat;
    background-position: center center;
    background-color: transparent;
    z-index: 9999;
}
.thumb-grid {
    display: block;
    width: 100%;
    padding: 0 0 0 0;
    margin: 5em 0 2.5em 0;
    list-style-type: none;
    background-color: transparent;
}
.thumb-grid:after {
    content:'';
    width: 0;
    display: block;
    clear: both;
}
.thumb-grid li {
    position: relative;
    cursor: pointer;
    overflow: hidden;
    width: 13%;
    margin: 0 8.75% 0 0;
    display: block;
    float: left;
    padding-bottom: 13%;
}
.thumb-grid li:nth-child(5n) {
    margin-right: 0;
}
.thumb-grid img {
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    filter: url("data:image/svg+xml;utf8,<svg xmlns=\'http://www.w3.org/2000/svg\'><filter id=\'grayscale\'><feColorMatrix type=\'matrix\' values=\'0.3333 0.3333 0.3333 0 0 0.3333 0.3333 0.3333 0 0 0.3333 0.3333 0.3333 0 0 0 0 0 1 0\'/></filter></svg>#grayscale");
    /* Firefox 3.5+ */
    filter: gray;
    /* IE6-9 */
    -webkit-filter: grayscale(100%);
    /* Chrome 19+ & Safari 6+ */
    -webkit-transition: all 0.2s ease-in-out;
       -moz-transition: all 0.2s ease-in-out;
        -ms-transition: all 0.2s ease-in-out;
         -o-transition: all 0.2s ease-in-out;
            transition: all 0.2s ease-in-out;
}
.thumb-grid li:hover img {
    opacity: 0.5;
}

<ul class="thumb-grid">
    <li id="gallery" class="play-icon">
        <img src="http://placehold.it/250x250" />
    </li>
    <li>
        <img src="http://placehold.it/250x250" />
    </li>
    <li>
        <img src="http://placehold.it/250x250" />
    </li>
    <li>
        <img src="http://placehold.it/250x250" />
    </li>
    <li>
        <img src="http://placehold.it/250x250" />
    </li>
</ul>