Re: [w3c/webcomponents] Allow attribute changes/child creation on custom element creation (#781)

There is intentional no way to accomplish this. Elements are supposed to accept arbitrary light DOM, and react to it, like all built-in elements do. They should not cause any light DOM children to "sprout into existence" upon creation/connection.

In other words, HTML elements in general, and custom elements in particular, are not meant as "template macros" that expand into larger portions of DOM. They are a single piece of DOM---an element---whose consumers control their children, attributes, etc.

This is, of course, why we introduce shadow DOM: so that element creators can add arbitrary shadow-children which are under the _element_'s control, instead of the element _consumer_'s control.

Hope this helps!

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