[css-color-4] new pair of system colors, fix w3c#6008

Author: svgeesus

css-color-4/Overview.bs

@@ -1807,9 +1807,14 @@ System Colors</h3>
 		<dd><span class="swatch" style="--color: fieldtext"></span>&nbsp;Text in input fields.
 
 		<dt><dfn>Highlight</dfn>
-		<dd><span class="swatch" style="--color: highlight"></span>&nbsp;Background of selected items/text.
+		<dd><span class="swatch" style="--color: highlight"></span>&nbsp;Background of selected text, for example from ::selection.
 		<dt><dfn>HighlightText</dfn>
-		<dd><span class="swatch" style="--color: highlighttext"></span>&nbsp;Text of selected items/text.
+		<dd><span class="swatch" style="--color: highlighttext"></span>&nbsp;Text of selected text.
+
+		<dt><dfn>SelectedItem</dfn>
+		<dd><span class="swatch" style="--color: highlight"></span>&nbsp;Background of selected items, for example a selected checkbox.
+		<dt><dfn>SelectedItemText</dfn>
+		<dd><span class="swatch" style="--color: highlighttext"></span>&nbsp;Text of selected items.
 
 		<dt><dfn>Mark</dfn>
 		<dd><span class="swatch" style="--color: mark"></span>&nbsp;Background of text that has been specially marked