New Article: Introduction to Custom Elements

Hi all,

I have drafted the latest article "Introduction to Custom Elements".
Could you kindly review this?

Thanks.
Second half of this paragraph (starting with "This way..." is a little hard to follow. Suggest something like:

"This gives you an empty prototype object with the basic HTML element feature set in its prototype chain. Add any functions and properties you want to the prototype object, then pass your prototype to `document.registerElement` as shown below:"

(This rephrasing doesn't give all of the details, but the example shows exactly how to pass the prototype. Feel free to ignore this comment if the rephrase doesn't seem like an improvement.)