Hi,
I understand your need. you were not talking about adding an HTTP header but a header in your web page.
it means that you have to modify your page but in a controlled way.
Example below: you will have to rewrite your response by adding needed style and contents in order to display Production Notification.
<!DOCTYPE html>
<html>
<head>
<style type="text/css">
body { margin:0; }
#titre { background: #fd0; width:100%; text-align:center; position: fixed; }
</style>
</head>
<body>
<div id="titre">PRODUCTION</div>
<br>contents bla bla bla<br>
</body>
</html>
If you show me your HTML application code I can help you to achieve your need.
regards,