Add UA style for contenteditable=plaintext-only

Fixes #11350.

Author: zcorpan

source

@@ -139717,6 +139717,11 @@ progress { appearance: auto; }</code></pre>
   the caret is in such an element, it prevents <span data-x="collapsing margins">margins from
   collapsing</span> through the element.</p>
 
+  <p>The following rules are also expected to apply, as <span>presentational hints</span>:</p>
+
+  <pre><code class="css">@namespace "http://www.w3.org/1999/xhtml";
+
+[contenteditable="plaintext-only" i] { white-space: pre-wrap; }</code></pre>
 
 
   <h4>Text rendered in native user interfaces</h4>