New Article: Introduction to Custom Elements

Hi all,

I have drafted the latest article "Introduction to Custom Elements".
Could you kindly review this?

Thanks.
the Type Extension => a Type Extension

Sentence is a little complicated. Might benefit from breaking it after the Element and starting a new sentence, something like this:

"... Type Extension Custom Element. To use the element, use the original tag and specify the custom tag name using the `is` attribute:"