Part I of this book provided an understanding of how a browser lays out a page by introducing normal flow, positioning, and stacking contexts. Part II described how to use this understanding to manipulate the properties that affect the aforementioned topics to control a page’s layout. This knowledge was then used to create small widgets with specific purposes that could be combined to create a larger UI widget, the dialog example. However, these widgets are not web components as described by the W3C. They were designed to function within the current context of the Web. Part III is all about true web components and the benefits they offer. It will cover the parts that comprise the W3C Web Components specification, templates, custom elements, the shadow DOM, and imports. As part of this learning process, in this section of the book we will take the dialog widget from Parts I and II and convert it to a bona fide web component, making it more semantic, declarative, encapsulated, consumable, and maintainable. More importantly, here we will truly extend the web platform for the first time.