<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.box {
width: 250px;
height: 250px;
border: 2px solid #000;
/* box-shadow: inset poziomo pionowo promień-rozmycia zasięg kolor; */
box-shadow: 5px 5px 4px #999;
/* box-shadow: 5px 5px */
/* box-shadow: inset 5px 5px 4px #999; */
/* box-shadow: inset -5px -5px 4px; */
}
body {
background: linear-gradient(red,blue 80%,red);
/* background: linear-gradient(to top,red,blue); */
/* background: repeating-linear-gradient(red,blue 25%); */
}
</style>
</head>
<body>
<div class="box"></div>
</body>
</html>