Revert recent changes to rendering of <slot>

As well as some of the corresponding changes to its directionality and language.

This reverts recent changes to the rendering section for the slot
element that I made recently in 1dc4c7557f5a4c879c1ce65b9cb59b91310bbf5
(PR #9796, fixing #3699). These changes had specified that all slot
elements, even those without a dir attribute, specify the direction and
unicode-bidi properties.

When I attempted to implement these changes, I discovered that they
broke a significant number of web-platform-tests, due to a pattern that
makes me suspect they would pose a compatibility problem. In
particular, specifying the CSS direction property for all slot elements
breaks inheritance of CSS direction into slotted elements, including
those in user-agent shadow DOM. (This makes a clearly visible change
when elements are implemented in terms of user-agent shadow DOM, which
probably is not intended.)

While I realize that the recommended practice is to specify the HTML dir
attribute rather than the CSS direction property, I believe the CSS
direction property is used enough that this is not acceptable.
(https://chromestatus.com/metrics/css/timeline/popularity/3 says the
property is used on around 40% of page loads. While the number of
interesting uses is probably substantially lower, it's also probably
still significant.)

source

@@ -13341,8 +13341,6 @@ Transport Protocol">HTTP&lt;/abbr> today.&lt;/p></code></pre> <!-- DO NOT REWRAP
    <dd><p>Use the value of that attribute.</p></dd>
 
    <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>
 
@@ -13718,7 +13716,8 @@ Transport Protocol">HTTP&lt;/abbr> today.&lt;/p></code></pre> <!-- DO NOT REWRAP
 
      <li><p>If <var>descendant</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>descendant</var>.</p></li>
+     directionality">directionality</span> of that <span>shadow root</span>'s <span
+     data-x="concept-DocumentFragment-host">host</span>.</p></li>
 
      <li><p>If <var>descendant</var> is not a <code>Text</code> node, then
      <span>continue</span>.</p></li>
@@ -13754,11 +13753,6 @@ 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> 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
@@ -131375,16 +131369,15 @@ 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), 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; }
+[dir]:dir(ltr), bdi:dir(ltr), input[type=tel i]:dir(ltr) { direction: ltr; }
+[dir]: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,
-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="" -->
+[dir=ltr i], [dir=rtl i], [dir=auto i] {
+  unicode-bidi: isolate; <!-- anything that's similar to display:block, plus <bdi>, <output>, 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 -->