New Article: Introduction to Custom Elements

Hi all,

I have drafted the latest article "Introduction to Custom Elements".
Could you kindly review this?

Thanks.
Somewhat complicated. Maybe use bullets?

To define a type extension element:

- Create the base prototype object using the prototype of the extended element, instead of `HTMLElement`.
- Add an `extends` key in the second argument to `document.registerElement()`, specifying the _tag name_ of the extended element.