Why does IPN validation return an HTML page instead of VERIFIED/INVALID?
IPN is designed to respond with a single word, either VERIFIED or INVALID, when the IPN data is sent (via POST) back to PayPal for validation. For a detailed description of this, see Receive payment notifications.
In rare cases, such as when the PayPal system is down or scheduled for maintenance, you may receive an HTML page instead. In this situation, you should log all the IPN data (the initial post you received) and try to validate the IPN data again later, when the PayPal system is back up.