Gradient Button.
<!DOCTYPE html>
<html>
<head>
	<title>Gradient</title>
	<link rel="stylesheet" type="text/css" href="style.css">
	<style type="text/css">
		body{
	margin: 0;
	padding: 0;
	background: #000;
}
a{
	position: absolute;
	top: 50%;
	left: 50%;
	transform: translate(-50% -50%);
	width: 200px;
	height: 60px;
	text-align: center;
	line-height: 60px;
	color: #fff;
	font-size: 24px;
	text-transform: uppercase;
	text-decoration: none;
	font-family: "Lucida Grande", "Lucida Sans Unicode", "Lucida Sans", "DejaVu Sans", Verdana, "sans-serif";
	box-sizing: border-box;
	background: linear-gradient(90deg,#03a9f4,#f441a5,#ffeb3b,#03a9f4);
	background-size: 400%;
	border-radius: 30px;
}
a:hover{
	animation: animate 8s linear infinite;
}
@keyframes animate{
	0%
	{
		background-position: 0%;
	}
	100%
	{
		background-position: 400%;
	}
}
a:before{
	content: '';
	position: absolute;
	top: -5px;
	left: -5px;
	right: -5px;
	bottom: -5px;
	z-index: -1px;
	background: linear-gradient(90deg,#03a9f4,#f441a5,#ffeb3b,#03a9f4);
	background-size: 400%;
	border-radius: 40px;
	opacity: 0;
	transition: 0.5s;
}
a:hover:before{
	filter: blur(20px);
	animation: animate 8s linear infinite;
}
	</style>
</head>
<body>
	<a href="#">Button</a>
</body>
</html>
