Dilettante_Pro,
вариант ...
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style>
.cont:hover div:hover ~ div, .cont div{
opacity: .2
}
.cont:hover div {
opacity: 1;
}
.cont {
display: flex;
}
div {
height:100px;
width:100px;
background:black;
}
</style>
</head>
<body>
<section class='cont'>
<div></div>
<div></div>
<div></div>
<div></div>
<div></div>
</section>
</body>
</html>