Un-customify adopted elements

Fixes whatwg/html#1359.

dom.bs

@@ -5036,10 +5036,23 @@ a <var>document</var>, run these steps:
 
   <ol>
    <li><p>For each <var>inclusiveDescendant</var> in <var>node</var>'s
-   <a>shadow-including inclusive descendants</a>, in <a>shadow-including tree order</a>, set
-   <var>inclusiveDescendant</var>'s <a>node document</a> to <var>document</var>.
+   <a>shadow-including inclusive descendants</a>, set <var>inclusiveDescendant</var>'s
+   <a>node document</a> to <var>document</var>.
    <!--AttrExodus as well as any associated {{Attr}} nodes-->
 
+   <li>
+    <p>For each <var>inclusiveDescendant</var> in <var>node</var>'s
+    <a>shadow-including inclusive descendants</a> that is an <a>element</a> with
+    <a>custom element state</a> of "<code>custom</code>", set <var>inclusiveDescendant</var>'s
+    <a>custom element state</a> to "<code>undefined</code>".</p>
+
+    <p class="note">
+     If this adoption is followed by an <a lt="insert" for="Node">insertion</a>, as most are, then
+     the insertion will <a lt="try to upgrade an element">try to upgrade</a>
+     <var>inclusiveDescendant</var>, potentially causing it to become <a>custom</a> again.
+    </p>
+   </li>
+
    <li><p>For each <var>inclusiveDescendant</var> in <var>node</var>'s
    <a>shadow-including inclusive descendants</a>, in <a>shadow-including tree order</a>, run the
    <a>adopting steps</a> with <var>inclusiveDescendant</var> and <var>oldDocument</var>.

dom.html

@@ -2730,7 +2730,7 @@ <h3 class="heading settled" data-level="4.4" id="interface-node"><span class="se
    </ol>
    <p>When invoked with the same argument, and as long as <var>root</var>’s <a data-link-type="dfn" href="#concept-node-document">node document</a>’s <a data-link-type="dfn" href="#concept-document-type">type</a> has not changed, the same <code class="idl"><a data-link-type="idl" href="#htmlcollection">HTMLCollection</a></code> object may be returned as
 returned by an earlier call. </p>
-   <p>The <dfn data-dfn-type="dfn" data-export="" id="concept-getelementsbytagnamens">list of elements with namespace <var>namespace</var> and local name <var>localName</var><a class="self-link" href="#concept-getelementsbytagnamens"></a></dfn> for a <a data-link-type="dfn" href="#concept-node">node</a> <var>root</var> is the <code class="idl"><a data-link-type="idl" href="#htmlcollection">HTMLCollection</a></code> returned by the following algorithm:</p>
+   <p>The <dfn data-dfn-type="dfn" data-export="" data-lt="list of elements with namespace namespace and local name localName" id="concept-getelementsbytagnamens">list of elements with namespace <var>namespace</var> and local name <var>localName</var><a class="self-link" href="#concept-getelementsbytagnamens"></a></dfn> for a <a data-link-type="dfn" href="#concept-node">node</a> <var>root</var> is the <code class="idl"><a data-link-type="idl" href="#htmlcollection">HTMLCollection</a></code> returned by the following algorithm:</p>
    <ol>
     <li>If <var>namespace</var> is the empty string, set it to null. 
     <li>If both <var>namespace</var> and <var>localName</var> are "<code>*</code>" (U+002A), return a <code class="idl"><a data-link-type="idl" href="#htmlcollection">HTMLCollection</a></code> rooted at <var>root</var>, whose filter matches <a data-link-type="dfn" href="#concept-tree-descendant">descendant</a> <a data-link-type="dfn" href="#concept-element">elements</a>. 
@@ -3025,7 +3025,11 @@ <h3 class="heading settled" data-level="4.5" id="interface-document"><span class
      <p>If <var>document</var> is not the same as <var>oldDocument</var>, run these substeps: </p>
      <ol>
       <li>
-       <p>For each <var>inclusiveDescendant</var> in <var>node</var>’s <a data-link-type="dfn" href="#concept-shadow-including-inclusive-descendant">shadow-including inclusive descendants</a>, in <a data-link-type="dfn" href="#concept-shadow-including-tree-order">shadow-including tree order</a>, set <var>inclusiveDescendant</var>’s <a data-link-type="dfn" href="#concept-node-document">node document</a> to <var>document</var>. </p>
+       <p>For each <var>inclusiveDescendant</var> in <var>node</var>’s <a data-link-type="dfn" href="#concept-shadow-including-inclusive-descendant">shadow-including inclusive descendants</a>, set <var>inclusiveDescendant</var>’s <a data-link-type="dfn" href="#concept-node-document">node document</a> to <var>document</var>. </p>
+      <li>
+       <p>For each <var>inclusiveDescendant</var> in <var>node</var>’s <a data-link-type="dfn" href="#concept-shadow-including-inclusive-descendant">shadow-including inclusive descendants</a> that is an <a data-link-type="dfn" href="#concept-element">element</a> with <a data-link-type="dfn" href="#concept-element-custom-element-state">custom element state</a> of "<code>custom</code>", set <var>inclusiveDescendant</var>’s <a data-link-type="dfn" href="#concept-element-custom-element-state">custom element state</a> to "<code>undefined</code>".</p>
+       <p class="note" role="note"> If this adoption is followed by an <a data-link-type="dfn" href="#concept-node-insert">insertion</a>, as most are, then
+     the insertion will <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/scripting.html#concept-try-upgrade">try to upgrade</a> <var>inclusiveDescendant</var>, potentially causing it to become <a data-link-type="dfn" href="#concept-element-custom">custom</a> again. </p>
       <li>
        <p>For each <var>inclusiveDescendant</var> in <var>node</var>’s <a data-link-type="dfn" href="#concept-shadow-including-inclusive-descendant">shadow-including inclusive descendants</a>, in <a data-link-type="dfn" href="#concept-shadow-including-tree-order">shadow-including tree order</a>, run the <a data-link-type="dfn" href="#concept-node-adopt-ext">adopting steps</a> with <var>inclusiveDescendant</var> and <var>oldDocument</var>. </p>
      </ol>
@@ -5980,8 +5984,7 @@ <h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="c
    <li><a href="#concept-light-tree">light tree</a><span>, in §4.2.2</span>
    <li><a href="#concept-document-limited-quirks">limited-quirks mode</a><span>, in §4.5</span>
    <li><a href="#concept-getelementsbyclassname">list of elements with class names classNames</a><span>, in §4.4</span>
-   <li><a href="#concept-getelementsbytagnamens">list of elements with namespace
-namespace and local name localName</a><span>, in §4.4</span>
+   <li><a href="#concept-getelementsbytagnamens">list of elements with namespace namespace and local name localName</a><span>, in §4.4</span>
    <li><a href="#concept-getelementsbytagname">list of elements with qualified name qualifiedName</a><span>, in §4.4</span>
    <li><a href="#concept-collection-live">live</a><span>, in §4.2.10</span>
    <li><a href="#concept-collection-live">live collection</a><span>, in §4.2.10</span>