What is API?
an API (or application programming interface) is “is a set of subroutine definitions, protocols, and tools for building application software.” … HTML APIs usually consist of certain class and attribute patterns that can be used on existing HTML.
In addition to specifying markup, HTML5 specifies scripting application programming interfaces (APIs) that can be used with JavaScript. Existing document object model (DOM) interfaces are extended and de facto features documented. There are APIs, such as: