The examples below build on the basic examples covered in the JavaScript: core concepts of the craft notes to create dynamic HTML/CSS content. Web browsers have built in "APIs" (application programming interfaces) which are collections of variables and functions that we can use to do all sorts of things. There are browser APIs for generating sounds, for getting data from other servers (referred to as 3rd-party "REST APIs"), for creating 2D graphics and 3D graphics, for creating Virtual and Augmented reality and so much more.
The most fundamental of these browser APIs is likely the DOM (document object model) API. When the browser renders our HTML page, behind the scenes it also creates a JavaScript Object named "document" which represents our entire HTML page, with all it's HTML and CSS. This special "document" Object has all sorts of built in properties (internal variables) and methods (internal functions) that enable us to interact with the HTML/CSS content on our page. The examples below demonstrate some of what's possible with this API, but you can learn more at Mozilla's developer docs on the DOM.