Add conformance requirements for custom element constructors

Fixes #409.

spec/custom/index.html

@@ -749,6 +749,19 @@ <h3>DOM+: New <a href="https://dom.spec.whatwg.org/#document"><code>Document</co
     <p>Notably, all the of the ways in which <a href="https://html.spec.whatwg.org/#the-button-element"><code>button</code></a> is special apply to such "plastic buttons" as well: their focus behavior, ability to participate in <a href="https://html.spec.whatwg.org/#form-submission-2">form submission</a>, the <a href="https://html.spec.whatwg.org/#attr-fe-disabled"><code>disabled</code></a> attribute, and so on.</p>
 </div>
 
+<p class="monkeypatch">This may belong in HTML, not DOM? Part of the semantics section?</p>
+
+<p>When authoring <a>custom element constructors</a>, developers are bound by the following <a href="https://html.spec.whatwg.org/#conformance-requirements-for-authors">conformance requirements</a>:</p>
+
+<ul>
+    <li>A parameter-less call to <code>super()</code> must be the first statement in the constructor body, to establish the correct prototype chain and <code>this</code> value before any further code is run.</li>
+    <li>A <code>return</code> statement must not appear anywhere inside the constructor body.</li>
+    <li>The element's attributes and children must not be inspected, as in the non-<a lt="upgrade a custom element">upgrade</a> case none will be present, and relying on upgrades makes the element less usable.</li>
+    <li>The element must not gain any attributes or children, as this violates the expectations of consumers who use the <a href="https://dom.spec.whatwg.org/#dom-document-createelement"><code>createElement</code></a> or <a href="https://dom.spec.whatwg.org/#dom-document-createelementns"><code>createElementNS</code></a> methods.</li>
+    <li>In general, work should be deferred to <code>attachedCallback</code> as much as possible—especially work involving fetching resources or rendering.</li>
+    <li>In general, the constructor should be used to set up initial state and default values.</li>
+</ul>
+
 </section>
 </section>
 <section id="instantiating-custom-elements">