:visible

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.0/jquery.min.js"></script>
</head>
<body>
<div id="main">
<p>Hello World!</p>
<p hidden="hidden">Привте Мир!</p>
<p>To be or not to be?</p>
<p hidden="hidden">Быть или не быть?</p>
<p>Lugansk is a city!</p>
</div>
<script>
$(document).ready(function() {
$('#main>:visible').css({ 'color': '#F00', 'font-size': '20px' });
});
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.0/jquery.min.js"></script>
</head>
<body>
<div id="main">
<p>Hello World!</p>
<p hidden="hidden">Привте Мир!</p>
<p>To be or not to be?</p>
<p hidden="hidden">Быть или не быть?</p>
<p>Lugansk is a city!</p>
</div>
<script>
$(document).ready(function() {
$('#main>:visible').css({ 'color': '#F00', 'font-size': '20px' });
});
</script>
</body>
</html>