Re: [webcomponents] Large custom element spec rewrite to implement some F2F decisions (#405)

> -    <li>If <var>PROTOTYPE</var> is <strong>null</strong>, let <var>PROTOTYPE</var> be the result of invoking <a href="https://tc39.github.io/ecma262/#sec-object.create"><code>Object.create</code></a> with <a href="https://html.spec.whatwg.org/multipage/dom.html#htmlelement"><code>HTMLElement</code></a>'s <a href="https://heycam.github.io/webidl/#dfn-interface-prototype-object">interface prototype object</a> as only argument</li>
> -    <li>Let <var>NAME</var> be <var>EXTENDS</var></li>
> -    <li>Let <var>ERROR</var> be the result of running the <a>element registration algorithm</a> with <var>DOCUMENT</var>, <var>TYPE</var>, <var>PROTOTYPE</var>, and <var>NAME</var> as arguments</li>
> -    <li>If <var>ERROR</var> is <code>InvalidType</code>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>SyntaxError</code> and <strong>stop</strong>.</li>
> -    <li>If <var>ERROR</var> is not <code>None</code>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> and <strong>stop</strong>.</li>
> -    <li>Return result of running <a data-lt="custom element constructor generation algorithm">custom element constructor generation algorithm</a> with <var>DOCUMENT</var> and <var>PROTOTYPE</var> as arguments.</li>
> +<ol>
> +    <li>If <a href="https://tc39.github.io/ecma262/#sec-isconstructor">IsConstructor</a>(<var>constructor</var>) is false, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>TypeError</code> and abort these steps.</li>
> +
> +    <li>If the <a href="https://dom.spec.whatwg.org/#context-object">context object</a> is an <a href="https://dom.spec.whatwg.org/#html-document">HTML document</a>, let <var>type</var> be <a href="https://dom.spec.whatwg.org/#converted-to-ascii-lowercase">converted to ASCII lowercase</a>.</li>
> +
> +    <li>If <var>type</var> is an invalid <a>custom element type</a>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>SyntaxError</code> and abort these steps.</li>
> +
> +    <li>Let <var>registry</var> be the <a href="https://dom.spec.whatwg.org/#context-object">context object</a>'s <a>registry</a>. If <var>registry</var> does not exist, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> and abort these steps.</li>
> +
> +    <li>If <var>registry</var> contains an entry with <a href="#dfn-element-definition-type">custom element type</a> <var>type</var>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> and abort these steps.</li>

We also need to check any entry with `constructor` since it seems that we don't want to allow a single interface to be associated with multiple tag names at least in v1.

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