The left pane of the designer is the Treeview pane. It contains a hierarchical representation of an HTML document’s contents. Each item in the HTML document—such as images, text, buttons, and so on—are called elements. Once elements are added, they can be manipulated during runtime through code. Each element listed in the Treeview pane includes information on the type of control, the beginning of its content (for elements such as text), and (when available) an ID. IDs are used to identify an element in code, and appear bolded in the TextView pane. IDs can be added or changed through the Properties window.