[css-ui] Add an issue about slider-vertical & sliderthumb-vertical (#3967)

Tests: already covered by css/css-ui/appearance-cssom-001.html

Part of #3024.

Author: zcorpan

css-ui-4/Overview.bs

@@ -1885,6 +1885,15 @@ so that CSS can be used to restyle them.
 
 </dl>
 
+<div class=issue>
+	The values <code>slider-vertical</code> and <code>sliderthumb-vertical</code>
+	are supported in some user agents to change the orientation of
+	<code class="lang-markup">&lt;input type=range></code> elements.
+	These values are not specified because changing the orientation of this control
+	is better done with a separate mechanism.
+	See issue <a href="https://github.com/whatwg/html/issues/4177">whatwg/html#4177</a>.
+</div>
+
 <div class=example>
 	An author wanting to customize the look and feel of check boxes in HTML could use the following:
 	<pre><code class="lang-css">