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    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW - Shadow DOM: Projected light DOM children do not transition"
   href="https://bugs.webkit.org/show_bug.cgi?id=160866">160866</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>Shadow DOM: Projected light DOM children do not transition
          </td>
        </tr>

        <tr>
          <th>Classification</th>
          <td>Unclassified
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>WebKit
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>Safari Technology Preview
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>Unspecified
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>Unspecified
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>Normal
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P2
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>HTML DOM
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>webkit-unassigned&#64;lists.webkit.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>webkit&#64;keanulee.com
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>cdumez&#64;apple.com
          </td>
        </tr></table>
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        <div>
        <pre>Repro: <a href="http://jsbin.com/jaqofi/2/edit?html,output">http://jsbin.com/jaqofi/2/edit?html,output</a>

As soon as a shadow root is added to an element, its children do not have CSS transitions. In this example:

    span {
      transition: background 2s;
    }
    body.green span {
      background: green;
    }

...

  &lt;button onclick=&quot;document.body.classList.toggle('green')&quot;&gt;Transition&lt;/button&gt;

  &lt;div&gt;
    &lt;span&gt;No shadow root&lt;/span&gt;
  &lt;/div&gt;
  &lt;div class=&quot;shadow&quot;&gt;
    &lt;span&gt;Projected in shadow root&lt;/span&gt;
  &lt;/div&gt;

...

  document.querySelector('div.shadow').attachShadow({mode:'open'}).innerHTML = '&lt;slot&gt;&lt;/slot&gt;';


the second span does not transition.</pre>
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