diff --git a/source b/source
index 9f6bfcc10b4..d75f1720c93 100644
--- a/source
+++ b/source
@@ -68241,6 +68241,7 @@ not-slash     = %x0000-002E / %x0030-10FFFF
    <dd><span>Global attributes</span></dd>
    <dd><code data-x="attr-template-shadowrootmode">shadowrootmode</code></dd>
    <dd><code data-x="attr-template-shadowrootdelegatesfocus">shadowrootdelegatesfocus</code></dd>
+   <dd><code data-x="attr-template-shadowrootslotassignment">shadowrootslotassignment</code></dd>
    <dd><code data-x="attr-template-shadowrootclonable">shadowrootclonable</code></dd>
    <dd><code data-x="attr-template-shadowrootserializable">shadowrootserializable</code></dd>
    <dd><code data-x="attr-template-shadowrootcustomelementregistry">shadowrootcustomelementregistry</code></dd>
@@ -68257,6 +68258,7 @@ interface <dfn interface>HTMLTemplateElement</dfn> : <span>HTMLElement</span> {
   readonly attribute <span>DocumentFragment</span> <span data-x="dom-template-content">content</span>;
   [<span>CEReactions</span>] attribute DOMString <span data-x="dom-template-shadowrootmode">shadowRootMode</span>;
   [<span>CEReactions</span>, <span data-x="xattr-Reflect">Reflect</span>] attribute boolean <dfn attribute for="HTMLTemplateElement" data-x="dom-template-shadowrootdelegatesfocus">shadowRootDelegatesFocus</dfn>;
+  [<span>CEReactions</span>] attribute DOMString <span data-x="dom-template-shadowrootslotassignment">shadowRootSlotAssignment</span>;
   [<span>CEReactions</span>, <span data-x="xattr-Reflect">Reflect</span>] attribute boolean <dfn attribute for="HTMLTemplateElement" data-x="dom-template-shadowrootclonable">shadowRootClonable</dfn>;
   [<span>CEReactions</span>, <span data-x="xattr-Reflect">Reflect</span>] attribute boolean <dfn attribute for="HTMLTemplateElement" data-x="dom-template-shadowRootSerializable">shadowRootSerializable</dfn>;
   [<span>CEReactions</span>, <span data-x="xattr-Reflect">Reflect</span>] attribute DOMString <dfn attribute for="HTMLTemplateElement" data-x="dom-template-shadowRootCustomElementRegistry">shadowRootCustomElementRegistry</dfn>;
@@ -68300,6 +68302,32 @@ interface <dfn interface>HTMLTemplateElement</dfn> : <span>HTMLElement</span> {
   data-x="attr-template-shadowrootdelegatesfocus">shadowrootdelegatesfocus</code></dfn> content
   attribute is a <span>boolean attribute</span>.</p>
 
+  <p>The <dfn element-attr for="template"><code
+  data-x="attr-template-shadowrootslotassignment">shadowrootslotassignment</code></dfn> content
+  attribute is an <span>enumerated attribute</span> with the following keywords and states:</p>
+
+  <table>
+   <thead>
+    <tr>
+     <th>Keyword
+     <th>State
+     <th>Brief description
+   <tbody>
+    <tr>
+     <td><dfn attr-value for="template/shadowrootslotassignment"><code data-x="attr-shadowrootslotassignment-named">named</code></dfn>
+     <td><dfn data-x="attr-shadowrootslotassignment-named-state">Named</dfn>
+     <td>The declarative shadow root uses named slot assignment.
+    <tr>
+     <td><dfn attr-value for="template/shadowrootslotassignment"><code data-x="attr-shadowrootslotassignment-manual">manual</code></dfn>
+     <td><dfn data-x="attr-shadowrootslotassignment-manual-state">Manual</dfn>
+     <td>The declarative shadow root uses manual slot assignment.
+  </table>
+
+  <p>The <code data-x="attr-template-shadowrootslotassignment">shadowrootslotassignment</code>
+  attribute's <i data-x="invalid value default">invalid value default</i> and <i data-x="missing
+  value default">missing value default</i> are both the <span
+  data-x="attr-shadowrootslotassignment-named-state">Named</span> state.</p>
+
   <p>The <dfn element-attr for="template"><code
   data-x="attr-template-shadowrootclonable">shadowrootclonable</code></dfn> content attribute is a
   <span>boolean attribute</span>.</p>
@@ -68465,6 +68493,14 @@ interface <dfn interface>HTMLTemplateElement</dfn> : <span>HTMLElement</span> {
   attribute, <span>limited to only known values</span>.</p>
   </div>
 
+  <div algorithm>
+  <p>The <dfn attribute for="HTMLTemplateElement"><code
+  data-x="dom-template-shadowrootslotassignment">shadowRootSlotAssignment</code></dfn> IDL attribute
+  must <span>reflect</span> the <code
+  data-x="attr-template-shadowrootslotassignment">shadowrootslotassignment</code> content attribute,
+  <span>limited to only known values</span>.</p>
+  </div>
+
   <p class="note">The <span
   data-x="dom-template-shadowRootCustomElementRegistry">shadowRootCustomElementRegistry</span> IDL
   attribute intentionally does not have a boolean type so it can be extended.</p>
@@ -141126,6 +141162,13 @@ document.body.appendChild(text);
        <li><p>Let <var>mode</var> be <var>templateStartTag</var>'s <code
        data-x="attr-template-shadowrootmode">shadowrootmode</code> attribute's value.</p></li>
 
+       <li><p>Let <var>slotAssignment</var> be "<code data-x="">named</code>".</p></li>
+
+       <li><p>If <var>templateStartTag</var>'s <code
+       data-x="attr-template-shadowrootslotassignment">shadowrootslotassignment</code> attribute is
+       in the <span data-x="attr-shadowrootslotassignment-manual-state">Manual</span> state, then
+       set <var>slotAssignment</var> to "<code data-x="">manual</code>".</p></li>
+
        <li><p>Let <var>clonable</var> be true if <var>templateStartTag</var> has a <code
        data-x="attr-template-shadowrootclonable">shadowrootclonable</code> attribute; otherwise
        false.</p></li>
@@ -141154,7 +141197,7 @@ document.body.appendChild(text);
          <li>
           <p><span data-x="concept-attach-a-shadow-root">Attach a shadow root</span> with
           <var>declarativeShadowHostElement</var>, <var>mode</var>, <var>clonable</var>,
-          <var>serializable</var>, <var>delegatesFocus</var>, "<code data-x="">named</code>", and
+          <var>serializable</var>, <var>delegatesFocus</var>, <var>slotAssignment</var>, and
           <var>registry</var>.</p>
 
           <p>If an exception is thrown, then catch it and:</p>
@@ -144946,6 +144989,10 @@ document.body.appendChild(text);
        <li><p>If <var>shadow</var>'s <span data-x="shadow-serializable">serializable</span> is set, then append
        "<code data-x=""> shadowrootserializable=&quot;&quot;</code>".</p></li>
 
+       <li><p>If <var>shadow</var>'s <span>slot assignment</span> is "<code
+       data-x="">manual</code>", then append "<code
+       data-x=""> shadowrootslotassignment=&quot;manual&quot;</code>".</p></li>
+
        <li><p>If <var>shadow</var>'s <span>clonable</span> is set, then append
        "<code data-x=""> shadowrootclonable=&quot;&quot;</code>".</p></li>
 
@@ -152880,6 +152927,7 @@ interface <dfn interface>External</dfn> {
      <td><span data-x="global attributes">globals</span>;
          <code data-x="attr-template-shadowrootmode">shadowrootmode</code>;
          <code data-x="attr-template-shadowrootdelegatesfocus">shadowrootdelegatesfocus</code>;
+         <code data-x="attr-template-shadowrootslotassignment">shadowrootslotassignment</code>;
          <code data-x="attr-template-shadowrootclonable">shadowrootclonable</code>;
          <code data-x="attr-template-shadowrootserializable">shadowrootserializable</code>;
          <code data-x="attr-template-shadowrootcustomelementregistry">shadowrootcustomelementregistry</code></td>
@@ -154438,6 +154486,12 @@ interface <dfn interface>External</dfn> {
      <td> <code data-x="attr-template-shadowrootserializable">template</code>
      <td> Sets <span data-x="shadow-serializable">serializable</span> on a declarative shadow root
      <td> <span>Boolean attribute</span>
+    <tr>
+     <th> <code data-x="">shadowrootslotassignment</code>
+     <td> <code data-x="attr-template-shadowrootslotassignment">template</code>
+     <td> Sets <span>slot assignment</span> on a declarative shadow root
+     <td> "<code data-x="">named</code>";
+          "<code data-x="">manual</code>"
     <tr>
      <th> <code data-x="">shape</code>
      <td> <code data-x="attr-area-shape">area</code>