State an initial selection direction

Closes #4541.

source

@@ -55052,12 +55052,15 @@ MIT Room 32-G524
   <p>The <span data-x="concept-textarea/input-selection">selections</span> of <code>input</code> and
   <code>textarea</code> elements have a <dfn>selection direction</dfn>, which is either "<code
   data-x="">forward</code>", "<code data-x="">backward</code>", or "<code data-x="">none</code>".
-  This direction is set when the user manipulates the selection. The exact meaning of the selection
-  direction depends on the platform. To <dfn>set the selection direction</dfn> of an element to a
-  given direction, update the element's <span>selection direction</span> to the given direction,
-  unless the direction is "<code data-x="">none</code>" and the platform does not support that
-  direction; in that case, update the element's <span>selection direction</span> to "<code
-  data-x="">forward</code>".</p>
+  The exact meaning of the selection direction depends on the platform. This direction is set when
+  the user manipulates the selection.  The initial <span>selection direction</span> must be "<code
+  data-x="">none</code>" if the platform supports that direction, or "<code
+  data-x="">forward</code>" otherwise.</p>
+
+  <p>To <dfn>set the selection direction</dfn> of an element to a given direction, update the
+  element's <span>selection direction</span> to the given direction, unless the direction is "<code
+  data-x="">none</code>" and the platform does not support that direction; in that case, update the
+  element's <span>selection direction</span> to "<code data-x="">forward</code>".</p>
 
   <div class="note">
    <p>On Windows, the direction indicates the position of the caret relative to