.parent{
	margin: 10px;
	padding: 10px;
	background-color: #eee;
	border: 1px solid #ccc;
}
.parent div{
	margin:10px;
	padding:10px;
	width:200px;
	height:50px;
	font-size:16px;
	color:white;
	background-color:#00a;
}
.parent1 :is(.child1,.child3){
	background-color:#0a0;
}
.parent2:is(.child1,.child3){
	background-color:#0a0;
}