Relative size unit[em]
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="styles.css" />
</head>
<body>
<div class="container">
<div class="box">
my name is mohit
</div>
</div>
</body>
</html>
<!-- css part -->
.cotainer{
font-size:10px;
}
/*Here container is a parent of box */
.box{
font-size:5em;
}