<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
#fff{
display: block;
margin-top: 10%;
}
</style>
</head>
<body>
<span>1</span>
<span id="fff">2</span>
<span>3</span>
</body>
</html>
работает и в хроме и в FF