New Article: Introduction to Custom Elements

Hi all,

I have drafted the latest article "Introduction to Custom Elements".
Could you kindly review this?

Thanks.
Number is inconsistent here. Either discuss one callback, or multiple callbacks: 

callback => callbacks (or "a lifecycle callback" if  you want to go singular throughout)
those functions = the functions (or "the function" if singular)
a property => properties (or "a property" is singular)

Singular is probably simpler here...