diff --git a/source b/source
index 5f3e26cc485..c99f8d6004d 100644
--- a/source
+++ b/source
@@ -13283,7 +13283,8 @@ Transport Protocol">HTTP&lt;/abbr> today.&lt;/p></code></pre> <!-- DO NOT REWRAP
   <ref>XML</ref></p>
 
   <p>If these attributes are omitted from an element, then the language of this element is the same
-  as the language of its parent element, if any.</p>
+  as the language of its parent element, if any (except for <code>slot</code> elements in a
+  <span>shadow tree</span>).</p>
 
   <p>The <code data-x="attr-lang">lang</code> attribute in no namespace may be used on any <span
   data-x="HTML elements">HTML element</span>.</p>
@@ -13314,28 +13315,36 @@ Transport Protocol">HTTP&lt;/abbr> today.&lt;/p></code></pre> <!-- DO NOT REWRAP
 
   <hr>
 
-  <p>To determine the <dfn export for="Node">language</dfn> of a node, user agents must look at the
-  nearest ancestor element (including the element itself if the node is an element) that has a <span
-  data-x="attr-xml-lang"><code data-x="">lang</code> attribute in the <span>XML
-  namespace</span></span> set or is an <span data-x="HTML elements">HTML element</span> and has a
-  <code data-x="attr-lang">lang</code> in no namespace attribute set. That attribute specifies the
-  language of the node (regardless of its value).</p>
-
-  <p>If both the <code data-x="attr-lang">lang</code> attribute in no namespace and the <span
-  data-x="attr-xml-lang"><code data-x="">lang</code> attribute in the <span>XML
-  namespace</span></span> are set on an element, user agents must use the <span
-  data-x="attr-xml-lang"><code data-x="">lang</code> attribute in the <span>XML
-  namespace</span></span>, and the <code data-x="attr-lang">lang</code> attribute in no namespace
-  must be <span data-x="ignore">ignored</span> for the purposes of determining the element's
-  language.</p>
+  <p>To determine the <dfn export for="Node">language</dfn> of a node, user agents must use the
+  first appropriate step in the following list:</p>
+
+  <dl class="switch">
+   <dt>If the node is an element that has a <span data-x="attr-xml-lang"><code data-x="">lang</code>
+   attribute in the <span>XML namespace</span></span> set</dt>
+   <dd><p>Use the value of that attribute.</p></dd>
+
+   <dt>If the node is an <span data-x="HTML elements">HTML element</span> and has a <code
+   data-x="attr-lang">lang</code> in no namespace attribute set.</dt>
+   <dd><p>Use the value of that attribute.</p></dd>
 
-  <p>If node's <span data-x="inclusive ancestor">inclusive ancestors</span> do not have either
-  attribute set, but there is a <span>pragma-set default language</span> set, then that is the
-  language of the node. If there is no <span>pragma-set default language</span> set, then language
-  information from a higher-level protocol (such as HTTP), if any, must be used as the final
-  fallback language instead. In the absence of any such language information, and in cases where the
-  higher-level protocol reports multiple languages, the language of the node is unknown, and the
-  corresponding language tag is the empty string.</p>
+   <dt>If the node's parent is a <span>shadow root</span></dt>
+   <dt>If the node is a <code>slot</code> element whose <span>root</span> is a <span>shadow
+   root</span></dt>
+   <dd><p>Use the <span>language</span> of that <span>shadow root</span>'s <span
+   data-x="concept-DocumentFragment-host">host</span>.</p></dd>
+
+   <!-- this assumes that if there's an ancestor element, the parent must be an element -->
+   <dt>If the node's <span>parent element</span> is not null</dt>
+   <dd><p>Use the <span>language</span> of that <span>parent element</span>.</p></dd>
+
+   <dt>Otherwise</dt>
+   <dd><p>If there is a <span>pragma-set default language</span> set, then that is the language of
+   the node. If there is no <span>pragma-set default language</span> set, then language information
+   from a higher-level protocol (such as HTTP), if any, must be used as the final fallback language
+   instead. In the absence of any such language information, and in cases where the higher-level
+   protocol reports multiple languages, the language of the node is unknown, and the corresponding
+   language tag is the empty string.</p></dd>
+  </dl>
 
   <p>If the resulting value is not a recognized language tag, then it must be treated as an unknown
   language having the given language tag, distinct from all other languages. For the purposes of
@@ -13551,7 +13560,17 @@ Transport Protocol">HTTP&lt;/abbr> today.&lt;/p></code></pre> <!-- DO NOT REWRAP
    <dd><p>Return '<span data-x="concept-rtl">rtl</span>'.</p></dd>
 
    <dt><span data-x="attr-dir-auto-state">auto</span></dt>
-   <dd><p>Return the <span>auto directionality</span> of <var>element</var>.</p></dd>
+   <dd>
+    <ol>
+     <li><p>Let <var>result</var> be the <span>auto directionality</span> of
+     <var>element</var>.</p></li>
+
+     <li><p>If <var>result</var> is null, then return the <span>parent directionality</span> of
+     <var>element</var>.</p></li>
+
+     <li><p>Return <var>result</var>.</p></li>
+    </ol>
+   </dd>
 
    <dt><span data-x="attr-dir-undefined-state">undefined</span></dt>
    <dd>
@@ -13594,26 +13613,62 @@ Transport Protocol">HTTP&lt;/abbr> today.&lt;/p></code></pre> <!-- DO NOT REWRAP
      <li><p>If <var>element</var>'s <span data-x="concept-fe-value">value</span> is not the empty
      string, then return '<span data-x="concept-ltr">ltr</span>'.</p></li>
 
-     <li><p>Return the <span>parent directionality</span> of <var>element</var>.</p></li>
+     <li><p>Return null.</p></li>
     </ol>
    </li>
 
-   <li><p>Let <var>codePoint</var> be null.</p></li>
+   <li>
+    <p>If <var>element</var> is a <code>slot</code> element whose <span>root</span> is a
+    <span>shadow root</span> and <var>element</var>'s <span>assigned nodes</span> are not empty:</p>
+
+    <ol>
+     <li>
+      <p><span data-x="list iterate">For each</span> node <var>child</var> of <var>element</var>'s
+      <span>assigned nodes</span>:</p>
+
+      <ol>
+       <li><p>Let <var>childDirection</var> be null.</p></li>
+
+       <li><p>If <var>child</var> is a <code>Text</code> node, then set <var>childDirection</var> to
+       the <span>text node directionality</span> of <var>child</var>.</p></li>
+
+       <li>
+        <p>Otherwise:</p>
+
+        <ol>
+         <li><p><span>Assert</span>: <var>child</var> is an <code>Element</code> node.</p></li>
+
+         <li><p>Set <var>childDirection</var> to the <span>auto directionality</span> of
+         <var>child</var>.</p></li>
+        </ol>
+       </li>
+
+       <li><p>If <var>childDirection</var> is not null, then return
+       <var>childDirection</var>.</p></li>
+      </ol>
+     </li>
+
+     <li><p>Return null.</p></li>
+    </ol>
+   </li>
 
    <li>
-    <p>For each <code>Text</code> node <var>text</var> that is a descendant of <var>element</var>,
-    in <span>tree order</span>:</p>
+    <p><span data-x="list iterate">For each</span> node <var>descendant</var> of
+    <var>element</var>'s <span data-x="descendant">descendants</span>, in <span>tree
+    order</span>:</p>
 
     <ol>
      <li>
-      <p>If <var>text</var> has an ancestor element that is a descendant of <var>element</var> and
-      is also one of</p>
+      <p>If <var>descendant</var> has an ancestor element that is a descendant of <var>element</var>
+      and is also one of</p>
 
       <ul class="brief">
        <li>a <code>bdi</code> element</li>
        <li>a <code>script</code> element</li>
        <li>a <code>style</code> element</li>
        <li>a <code>textarea</code> element</li>
+       <li>a <code>slot</code> element whose <span>root</span> is a <span>shadow
+       root</span></li>
        <li>an element whose <code data-x="attr-dir">dir</code> attribute is not in the <span
        data-x="attr-dir-undefined-state">undefined</span> state</li>
       </ul>
@@ -13621,20 +13676,43 @@ Transport Protocol">HTTP&lt;/abbr> today.&lt;/p></code></pre> <!-- DO NOT REWRAP
       <p>then <span>continue</span>.</p>
      </li>
 
-     <li><p>If <var>text</var>'s <span data-x="concept-cd-data">data</span> does not contain a code
-     point whose bidirectional character type is L, AL, or R, then <span>continue</span>.
-     <ref>BIDI</ref></p></li>
+     <li>
+      <p>If <var>descendant</var> is a <code>slot</code> element whose <span>root</span> is a
+      <span>shadow root</span>, then:</p>
 
-     <li><p>Set <var>codePoint</var> to the first code point in <var>text</var>'s <span
-     data-x="concept-cd-data">data</span> whose bidirectional character type is L, AL, or
-     R.</p></li>
+      <ol>
+       <li><p>Let <var>result</var> be the <span>auto directionality</span> of
+       <var>descendant</var>.</p></li>.
+
+       <li><p>If <var>result</var> is not null, then return <var>result</var>.</p></li>
 
-     <li><p><span>Break</span>.</p></li>
+       <li><p>Return '<span data-x="concept-ltr">ltr</span>'.</p></li>
+      </ol>
+     </li>
+
+     <li><p>If <var>descendant</var> is not a <code>Text</code> node, then
+     <span>continue</span>.</p></li>
+
+     <li><p>Let <var>result</var> be the <span>text node directionality</span> of
+     <var>descendant</var>.</li>
+
+     <li><p>If <var>result</var> is not null, then return <var>result</var>.</p></li>
     </ol>
    </li>
 
-   <li><p>If <var>codePoint</var> is null, then return the <span>parent directionality</span> of
-   <var>element</var>.</p></li>
+   <li><p>Return null.</p></li>
+  </ol>
+
+  <p>To compute the <dfn>text node directionality</dfn> given a <code>Text</code> node
+  <var>text</var>:</p>
+
+  <ol>
+   <li><p>If <var>text</var>'s <span data-x="concept-cd-data">data</span> does not contain a code
+   point whose bidirectional character type is L, AL, or R, then return null.
+   <ref>BIDI</ref></p></li>
+
+   <li><p>Let <var>codePoint</var> be the first code point in <var>text</var>'s <span
+   data-x="concept-cd-data">data</span> whose bidirectional character type is L, AL, or R.</p></li>
 
    <li><p>If <var>codePoint</var> is of bidirectional character type AL or R, then return '<span
    data-x="concept-rtl">rtl</span>'.</p></li>
@@ -13646,11 +13724,21 @@ Transport Protocol">HTTP&lt;/abbr> today.&lt;/p></code></pre> <!-- DO NOT REWRAP
   <p>To compute the <dfn>parent directionality</dfn> given an element <var>element</var>:</p>
 
   <ol>
-   <li><p>If <var>element</var> has no <span>parent element</span>, then return '<span
-   data-x="concept-ltr">ltr</span>'.</p></li>
+   <li><p>If <var>element</var> is a <code>slot</code> element whose <span>root</span> is a
+   <span>shadow root</span>, then return the <span data-x="the directionality">directionality</span>
+   of <var>element</var>'s <span>root</span>'s <span
+   data-x="concept-DocumentFragment-host">host</span>.</p></li>
+
+   <li><p>Let <var>parentNode</var> be <var>element</var>'s parent node.</p></li>
+
+   <li><p>If <var>parentNode</var> is a <span>shadow root</span>, then return the <span data-x="the
+   directionality">directionality</span> of <var>parentNode</var>'s <span
+   data-x="concept-DocumentFragment-host">host</span>.</p></li>
+
+   <li><p>If <var>parentNode</var> is an element, then return the <span data-x="the
+   directionality">directionality</span> of <var>parentNode</var>.</p></li>
 
-   <li><p>Return <span data-x="the directionality">directionality</span> of <var>element</var>'s
-   <span data-x="parent element">parent</span>.</p></li>
+   <li><p>Return '<span data-x="concept-ltr">ltr</span>'.</p></li>
   </ol>
 
   <div w-nodev>
@@ -129391,15 +129479,16 @@ br[clear=all i], br[clear=both i] { clear: both; }</code></pre>
 
   <pre><code class="css">@namespace "http://www.w3.org/1999/xhtml";
 
-[dir]:dir(ltr), bdi:dir(ltr), input[type=tel i]:dir(ltr) { direction: ltr; }
-[dir]:dir(rtl), bdi:dir(rtl) { direction: rtl; }
+[dir]:dir(ltr), slot:dir(ltr), bdi:dir(ltr), input[type=tel i]:dir(ltr) { direction: ltr; }
+[dir]:dir(rtl), slot:dir(rtl), bdi:dir(rtl) { direction: rtl; }
 
 address, blockquote, center, div, figure, figcaption, footer, form, header, hr,
 legend, listing, main, p, plaintext, pre, summary, xmp, article, aside, h1, h2,
 h3, h4, h5, h6, hgroup, nav, section, search, table, caption, colgroup, col,
 thead, tbody, tfoot, tr, td, th, dir, dd, dl, dt, menu, ol, ul, li, bdi, output,
-[dir=ltr i], [dir=rtl i], [dir=auto i] {
-  unicode-bidi: isolate; <!-- anything that's similar to display:block, plus <bdi>, <output>, and dir="" -->
+slot, [dir=ltr i], [dir=rtl i], [dir=auto i] {
+  unicode-bidi: isolate; <!-- anything that's similar to display:block, plus
+                              <bdi>, <output>, <slot>, and dir="" -->
 }
 
 bdo, bdo[dir] { unicode-bidi: isolate-override; } <!-- bdo[dir] rule is to override the otherwise higher-specificity attribute selectors in the previous rule -->