:focus

css/style.css

input:focus {
background-color: #CCFFFF;
}

input:focus {
 background-color: #CCFFFF;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
</head>
<body>
<input type="text" />
<br />
<br />
<input type="text" />
<br />
<br />
<input type="submit" value="OK" />
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
</head>
<body>
<input type="text" />
<br />
<br />
<input type="text" />
<br />
<br />
<input type="submit" value="OK" />
</body>
</html>