Explain what is Shadow DOM in Polymer.js?

DOM stands for Document Object Model. It is a property of DOM used to building components. One can think of shadow DOM as a scoped sub-tree inside the Polymer.js element. Browsers use Shadow DOM to implement default HTML components such as the <video> tag. The Shadow DOM specification allows the developers to use the same techniques in our own custom elements. DOM is basically an application-programming interface (API) for valid HTML and well-formed XML documents. It defines the logical structure of documents and the way a document is accessed and manipulated. Shadow DOM, in particular, refers to the ability of the browser to include a sub-tree of DOM elements into the rendering of a document, but not into the main document DOM tree.