HTML being returned by a server is an API, i.e. an Application Programming Interface, it's an API designed to be consumed by a hypermedia client, e.g. a browser.
Quoting Roy Fielding:
"The design of the Web had to shift from the development of a
reference protocol library to the development of a network-based API, extending the desired semantics of the Web across multiple platforms and implementations.
A network-based API is an on-the-wire syntax, with defined semantics, for application interactions. A network-based API does not place any restrictions on the application code aside from the need to read/write to the network, but does place restrictions on the set of semantics that can be effectively communicated across the interface."
Quoting Roy Fielding:
"The design of the Web had to shift from the development of a reference protocol library to the development of a network-based API, extending the desired semantics of the Web across multiple platforms and implementations. A network-based API is an on-the-wire syntax, with defined semantics, for application interactions. A network-based API does not place any restrictions on the application code aside from the need to read/write to the network, but does place restrictions on the set of semantics that can be effectively communicated across the interface."