Interface used by open source COTS web applications

I am creating a web application which is used for providing the health and availability parameters of a server to external users.

As there is a possibility of this web application being replaced by an open source COTS software solution (such as Nagios) in the future, I would like to ensure that the interface between the web application and the server uses an open standard instead of a custom one.

What kind of open standards/ interfaces are used by open source COTS web applications?