The means Haiku runs anywhere your code does, on any modern browser.
In the simplest case, you don't even have to think about the "core" — we do the heavy lifting and provide the code snippets to include your Haiku in codebases of different kinds (see Publishing and Embedding). One line of code and done.
But sometimes you want more control: Maybe you want to control the timeline based on events, or change the way your component is sized inside its container — or any number of programmatic actions. For these cases, you can use the Haiku Core Developer API.