DOM types are available by default - no need to import anything.
Source code: https://github.com/microsoft/TypeScript/blob/main/lib/lib.dom.d.ts
Reference: https://developer.mozilla.org/en-US/docs/Web/API
Tutor: https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Client-side_web_APIs
Document - type of global "document"
looking for elements
getElementById
HTMLElement & HTMLxxxElement - various HTML elements
Event (and more specific xxxEvent) - "an event which takes place in the DOM"
target - EventTarget, should be casted into something more specific (HTMLElement or a more specific HTMLxxxElement)