Add Imperative Slot API

dom.bs

@@ -2195,6 +2195,10 @@ steps:</p>
  <li><p>If the <i>open flag</i> is set and <var>shadow</var>'s <a for=ShadowRoot>mode</a> is
  <em>not</em> "<code>open</code>", then return null.</p></li>
 
+ <li><p>If the <var>shadow</var>'s <a for=ShadowRoot>slot assignment</a> is set to "<code>manual</code>",
+ return the associated slot in <var>shadow</var>'s tree whose <a>manuallyAssignedNodes</a> includes
+ <var>slottable</var>, if any, and null otherwise.</p></li>
+
  <li><p>Return the first <a>slot</a> in <a>tree order</a> in <var>shadow</var>'s
  <a for=tree>descendants</a> whose <a for=slot>name</a> is <var>slottable</var>'s
  <a for=slottable>name</a>, if any, and null otherwise.</p></li>
@@ -5711,11 +5715,13 @@ invoked, must return a new {{DocumentFragment}} <a>node</a> whose <a for=Node>no
 [Exposed=Window]
 interface ShadowRoot : DocumentFragment {
   readonly attribute ShadowRootMode mode;
+  readonly attribute SlotAssignmentMode slotAssignment;
   readonly attribute Element host;
   attribute EventHandler onslotchange;
 };
 
 enum ShadowRootMode { "open", "closed" };
+enum SlotAssignmentMode { "manual", "auto" };
 </pre>
 
 <p>{{ShadowRoot}} <a for=/>nodes</a> are simply known as
@@ -5731,6 +5737,9 @@ It is initially set to false.</p>
 <!-- If we ever change this, e.g., add a ShadowRoot object constructor, that would have serious
      consequences for innerHTML. -->
 
+<p><a for=/>Shadow roots</a> have an associated <dfn for=ShadowRoot>slot assignment</dfn>
+("<code>manual</code>" or "<code>auto</code>"), whose default value is "<code>auto</code>".</p>
+
 <p>A <a for=/>shadow root</a>'s <a>get the parent</a> algorithm, given an <var>event</var>, returns
 null if <var>event</var>'s <a>composed flag</a> is unset and <a for=/>shadow root</a> is the
 <a for=tree>root</a> of <var>event</var>'s <a for=Event>path</a>'s first struct's
@@ -5748,6 +5757,9 @@ null if <var>event</var>'s <a>composed flag</a> is unset and <a for=/>shadow roo
 <dfn for=ShadowRoot export><code>onslotchange</code></dfn> <a>event handler</a>, whose
 <a>event handler event type</a> is {{HTMLSlotElement/slotchange}}.
 
+<p>The <dfn attribute for=ShadowRoot><code>slotAssignment</code></dfn> attribute's getter must
+return <a>this</a>'s <a for=ShadowRoot>slot assignment</a>.</p>
+
 <hr>
 
 <p>In <dfn export id=concept-shadow-including-tree-order>shadow-including tree order</dfn> is
@@ -5871,6 +5883,7 @@ interface Element : Node {
 dictionary ShadowRootInit {
   required ShadowRootMode mode;
   boolean delegatesFocus = false;
+  SlotAssignmentMode slotAssignment = "auto";
 };
 </pre>
 
@@ -6744,6 +6757,9 @@ invoked, must run these steps:
  <li><p>Set <var>shadow</var>'s <a for=ShadowRoot>delegates focus</a> to <var>init</var>'s
  {{ShadowRootInit/delegatesFocus}}.
 
+ <li><p>Set <var>shadow</var>'s <a for=ShadowRoot>slot assignment</a> to <var>init</var>'s
+ {{ShadowRootInit/slotAssignment}}.
+
  <li><p>Set <a>this</a>'s <a for=Element>shadow root</a> to <var>shadow</var>.
 
  <li><p>Return <var>shadow</var>.
@@ -10187,7 +10203,8 @@ Xidorn Quan,
 Yehuda Katz,
 Yoav Weiss,
 Yoichi Osato,
-Yoshinori Sano, and
+Yoshinori Sano,
+Yu Han, <!-- yuzhe-han on GitHub --> and
 Zack Weinberg
 for being awesome!