body,html {
	display: block;
	width: 100%;
	height: 100%;
	background: #000;
	padding: 0;
	margin: 0;
}

nav a,
nav a::before{
	position: relative;
	display: block;
	padding: 1em;
	color: #fff;
	font-size: 3em;
	overflow: hidden;
	background: transparent;
	text-align: center;
}

nav a:hover{
	background: #fff;
	color:#000;
}

nav a::before{
	position: absolute;
	display: block;
	content: "";
	color: #000;
	font-size: 1em;
	top: 0;
	left: 0;
	width: 100%;
	height: 100%;
	padding-left: 0;
	padding-right: 0;
	background: no-repeat center center;
	background-size: 100% auto;
	opacity: 0;
	transition:background-size 2s,
	opacity .5s;
}

nav a:hover::before{
	opacity: 1;
	background-size: 150% auto;
}