Add checks on custom element constructor return values

Closes #412.

spec/custom/index.html

@@ -255,6 +255,9 @@ <h3 id="custom-element-conformance">Requirements for custom element constructors
     <li>In general, work should be deferred to <code>connectedCallback</code> as much as possible—especially work involving fetching resources or rendering. However, note that <code>connectedCallback</code> can be called more than once, so any initialization work that is truly one-time will need a guard to prevent it from running twice.</li>
     <li>In general, the constructor should be used to set up initial state and default values, and to set up <a href="https://dom.spec.whatwg.org/#concept-event-listener">event listeners</a>.</li>
 </ul>
+
+<p>Several of these requirements are checked during element creation, either directly or indirectly, and failing to follow them will result in a custom element that cannot be instantiated by the parser or DOM APIs.</p>
+
 </section>
 
 <section>
@@ -531,17 +534,17 @@ <h3>DOM+: Elements</h3>
         <p>If <var>typeExtension</var> is not null:</p>
 
         <ol>
-            <li>If <var>registry</var> is null, <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> is null, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> exception.</li>
 
-            <li>If <var>namespace</var> is not the <a href="https://dom.spec.whatwg.org/#html-namespace">HTML namespace</a>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>TypeError</code> exception and abort these steps.</li>
+            <li>If <var>namespace</var> is not the <a href="https://dom.spec.whatwg.org/#html-namespace">HTML namespace</a>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>TypeError</code> exception.</li>
 
             <li>Let <var>interface</var> be the <a href="https://dom.spec.whatwg.org/#concept-element-interface">element interface</a> for <var>localName</var> and the <a href="https://dom.spec.whatwg.org/#html-namespace">HTML namespace</a>.</li>
 
             <li>
                 <p>If there is an the entry in <var>registry</var> with <a href="#dfn-element-definition-name">name</a> <var>typeExtension</var>, let <var>definition</var> be that entry, and perform the following subsubsteps:</p>
 
                 <ol>
-                    <li>If <var>definition</var>'s <a href="#dfn-element-definition-local-name">local name</a> is not <var>localName</var>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>TypeError</code> exception and abort these steps.</li>
+                    <li>If <var>definition</var>'s <a href="#dfn-element-definition-local-name">local name</a> is not <var>localName</var>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>TypeError</code> exception.</li>
 
                     <li>Let <var>result</var> be a new <a href="https://dom.spec.whatwg.org/#concept-element">element</a> that implements <var>interface</var>, with no attributes, <a href="https://dom.spec.whatwg.org/#concept-element-namespace">namespace</a> set to the <a href="https://dom.spec.whatwg.org/#html-namespace">HTML namespace</a>, <a href="https://dom.spec.whatwg.org/#concept-element-namespace-prefix">namespace prefix</a> set to <var>prefix</var>, <a href="https://dom.spec.whatwg.org/#concept-element-local-name">local name</a> set to <var>localName</var>, <a>defined flag</a> unset, and <a href="https://dom.spec.whatwg.org/#concept-node-document">node document</a> set to <var>document</var>.</li>
 
@@ -582,11 +585,27 @@ <h3>DOM+: Elements</h3>
                     <li>Let <var>result</var> be <a href="https://tc39.github.io/ecma262/#sec-construct">Construct</a>(<var>C</var>). Rethrow any exceptions.</li>
 
                     <li>
-                        <p>If <var>result</var> does not implement the <a href="https://html.spec.whatwg.org/multipage/dom.html#htmlelement"><code>HTMLElement</code></a> interface, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>TypeError</code> exception and abort these steps.</p>
+                        <p>If <var>result</var> does not implement the <a href="https://html.spec.whatwg.org/multipage/dom.html#htmlelement"><code>HTMLElement</code></a> interface, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>TypeError</code> exception.</p>
 
                         <p class="note">This is meant to be a brand check to ensure that the object was allocated by the <a href="#dom-htmlelement-constructor"><code>HTMLElement</code></a> constructor. Eventually Web IDL may give us a more precise way to do brand checks.</p>
                     </li>
 
+                    <li>If the value of <var>result</var>'s [[\Prototype]] internal slot is not equal to <var>definition</var>'s <a href="#dfn-element-definition-prototype">prototype</a>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>TypeError</code> exception.</li>
+
+                    <li>If <var>result</var>'s <a href="https://dom.spec.whatwg.org/#concept-element-attribute">attribute list</a> is not empty, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> exception.</li>
+
+                    <li>If <var>result</var> has <a href="https://dom.spec.whatwg.org/#concept-tree-child">children</a>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> exception.</li>
+
+                    <li>If <var>result</var>'s <a href="https://dom.spec.whatwg.org/#concept-tree-parent">parent</a> is not null, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> exception.</li>
+
+                    <li>If <var>result</var>'s <a href="https://dom.spec.whatwg.org/#concept-node-document">node document</a> is not <var>document</var>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> exception.</li>
+
+                    <li>If <var>result</var>'s <a href="https://dom.spec.whatwg.org/#concept-element-namespace">namespace</a> is not the <a href="https://dom.spec.whatwg.org/#html-namespace">HTML namespace</a>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> exception.</li>
+
+                    <li>If <var>result</var>'s <a href="https://dom.spec.whatwg.org/#concept-element-local-name">local name</a> is not <var>localName</var>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> exception.</li>
+
+                    <li>Set <var>result</var>'s <a href="https://dom.spec.whatwg.org/#concept-element-namespace-prefix">namespace prefix</a> to <var>prefix</var>.</li>
+
                     <li>Set <var>result</var>'s <a>defined flag</a>.</li>
 
                     <li>Return <var>result</var>.</li>