[css-pseudo] Add ::marker per WG resolution.

Author: fantasai

css-pseudo/Overview.bs

@@ -661,6 +661,24 @@ Generated Content Pseudo-elements: ''::before'' and ''::after''</h3>
   (<code>:before</code> and <code>:after</code>)
   for these pseudo-elements.
 
+<h3 id="marker-pseudo">
+List Markers: the ''::marker'' pseudo-element</h3>
+
+  The <dfn>::marker</dfn> pseudo-element represents
+  the automatically generated marker box of a list item.
+  (See ''display: list-item''.)
+
+  The following CSS properties apply to a ''::first-line'' pseudo-element:
+  <ul>
+    <li>all font properties (see [[CSS3-FONTS]])
+    <li>the 'color' property (see [[CSS3COLOR]])
+  </ul>
+
+  <p class="note">
+    It is expected that future specifications will extend this list of properties;
+    however at the moment marker box layout is not fully defined,
+    so only these properties are allowed.
+
 <h3 id="placeholder-pseudo">
 Placeholder Input: the ''::placeholder'' pseudo-element</h3>