I am moving this gist to a github repo so more people can contribute to it. Also, it makes it easier for me to version control.
Please go to - https://github.com/praveenpuglia/shadow-dom-in-depth for latest version of this document.
Also, if you find the document useful, please shower your love, go
Heads Up! It's all about the V1 Spec.
In a nutshell, Shadow DOM enables local scoping for HTML & CSS.