The structure of a web page is represented as a tree of nodes in the Document Object Model (DOM). A web page can start its life with an initial DOM tree, marked up in its HTML file, and then the tree can be changed using code; or, it can start off with an empty tree, which is then entirely created using code in the app, that is every element is created through a constructor and its properties are set in code. Elements are subclasses of Node; they take up a rectangular space on the web page (with a width and height). They have, at most, one parent Element in which they are enclosed and can contain a list of Elements—their children (you can check this with the function hasChildNodes() that returns a bool function). Furthermore, they can receive events. Elements must first be created before they can be added to the list of a parent element. Elements can also be removed from a node. When elements are added or removed, the DOM tree is changed and the browser has...