[css-forms-1] Add new pseudo-elements for customizable select (#10986)

Author: josepharhar

css-forms-1/Overview.bs

@@ -55,6 +55,33 @@ Styling form control pickers: the ''::picker()'' pseudo-element</h3>
 &lt;/style>
   </pre>
 
+<h3 id="picker-icon">
+Picker opener icon: the ''::picker-icon'' pseudo-element</h3>
+
+  The ''::picker-icon'' pseudo-element is only generated when the
+  <a>originating element</a> has <a>base appearance</a> and opens a picker.
+
+  ''::picker-icon'' is a [=tree-abiding pseudo-element=]. It accepts all
+  properties. It inherits from its [=originating element=].
+
+  ''::picker-icon'' 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="checkmark">
+Checkmark icon: the ''::checkmark'' pseudo-element</h3>
+
+  The '::checkmark'' pseudo-element is only generated when the <a>originating
+  element</a> supports the <a>:checked</a> pseudo-class and either has <a>base
+  appearance</a> or an ancestor with <a>base appearance</a>.
+
+  ''::checkmark'' is a [=tree-abiding pseudo-element=]. It accepts all properties.
+  It inherits from its [=originating element=].
+
+  ''::checkmark'' 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'.
+
 
 Basic Styling Proposals {#ideas}
 ================================