[css-forms-1] Add new pseudo-elements for customizable select

css-pseudo-4/Overview.bs

@@ -1634,6 +1634,34 @@ Styling form control pickers: the ''::picker()'' pseudo-element</h3>
 &lt;/style>
   </pre>
 
+<h3 id="select-arrow">
+Select dropdown icon: the ''::select-arrow'' pseudo-element</h3>
+
+  The ''::select-arrow'' pseudo-element is only generated when the
+  <a>originating element</a> is a <{select}> element with <a>base
+  appearance</a>.
+
+  ''::select-arrow'' is a [=tree-abiding pseudo-element=]. It accepts all
+  properties. It inherits from its [=originating element=].
+
+  ''::select-arrow'' generates a box as if it was an child of its
+  <a>originating element</a>, after any boxes generated by the ''::after''
+  pseudo-element, with content as specified by 'content'.
+
+<h3 id="check">
+Option checkmark icon: the ''::check'' pseudo-element</h3>
+
+  The '::check'' pseudo-element is only generated when the <a>originating
+  element</a> is a <{option}> element which has an ancestor <{select}> element
+  with <a>base appearance</a>.
+
+  ''::check'' is a [=tree-abiding pseudo-element=]. It accepts all properties.
+  It inherits from its [=originating element=].
+
+  ''::check'' generates a box as if it was an child of its <a>originating
+  element</a>, preceding any boxes generated by the ''::before''
+  pseudo-element, with content as specified by 'content'.
+
 
 <h2 id="interactions">
 Overlapping Pseudo-element Interactions</h2>
@@ -1986,4 +2014,4 @@ Acknowledgements</h2>
     get-computed-style-crash.html
     svg-text-selection-002.html
     text-selection.html
-  </wpt>
+  </wpt>