Re: [w3c/webcomponents] Support Custom Pseudo-elements (#300)

>From my point of view, custom elements are here to give authors the same power as user-agents have of defining elements, or at least as close to that as possible.

The elements defined by the spec are not necessarily implemented with JavaScript and CSS, but they could be. Even the most convoluted ones like `input` with all its types and shapes.

Giving authors the ability to create custom elements is putting them at nearly the same level as the user-agent, allowing them to define elements in the same way the user-agent does.

Not without giving authors the power to create custom pseudo-elements feels incomplete to me, as it’s yet another thing the user-agent can do that the author cannot. By giving authors the ability to create custom pseudo-elements you are bringing their power closer to the user-agent’s.

@tabatkins effectively, what I’m saying is that I prefer custom pseudo-elements to your suggestion because it is closer to what is done by user-agents to regular elements.

I suggest the following syntax:

```CSS
x-foo..custom-element
{
 /* styles */
}
```

As it’s comparable to what we have today:

```CSS
/* user-agent-defined class */
button:enabled
{}

/* author-defined class */
button.loading
{}

/* user-agent-defined pseudo-element */
x-foo::first-line
{}

/* author-defined pseudo-element */
x-foo..last-letter
{}
```

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