img {
	display:inline-block;
	width:256px;
	height:192px;
	/* 隐藏Firefox alt文字 */
    color:transparent;
	position:relative;
	overflow:hidden;
}
img:not([src]) {
	/* 隐藏Chrome alt文字以及银色边框 */
    visibility:hidden;
}
img::before {
	/* 淡蓝色占位背景 */
    content:"";
	position:absolute;
	left:0;
	width:100%;
	height:100%;
	background-color:#f0f3f9;
	visibility:visible;
}
img::after {
	/* 黑色alt信息条 */
    content:attr(alt);
	position:absolute;
	left:0;
	bottom:0;
	width:100%;
	line-height:30px;
	background-color:rgba(0,0,0,.5);
	color:white;
	font-size:14px;
	transform:translateY(100%);
	/* 来点过渡动画效果 */
    transition:transform .2s;
	visibility:visible;
}
img:hover::after {
	transform:translateY(0);
}