鼠标通过图片变暗，css

<div class="slidervideo">
     <a href="http://www.youtube.com/embed/dxLtlumu8X4" target="_blank"><img src="images/freezevideo.png" /></a>
</div>

css代码：

.slidervideo a{
    z-index:99999999999;
    opacity:0.7;
    filter:alpha(opacity=70);
}
.slidervideo a:hover{
    opacity:1;
    filter:alpha(opacity=100);
}