[selectors-4] Make :placeholder-shown clearly apply also when placeholder option lebel is shown per HTML. #2517

Author: fantasai

selectors-4/Overview.bs

@@ -2494,7 +2494,18 @@ The Placeholder-shown Pseudo-class: '':placeholder-shown''</h4>
 	as a hint to the user on what to type in.
 	See, for example, the <code>placeholder</code> attribute in [[HTML5]].
 	The <dfn id="placeholder-shown-pseudo">:placeholder-shown</dfn> pseudo-class
-	matches an input element that is showing such placeholder text.
+	matches an input element that is showing such placeholder text,
+	whether that text is given by an attribute or a real element,
+	or is otherwise implied by the UA.
+
+	<div class="example">
+		For example, according to the semantics of [[HTML]]
+		the <{input/placeholder}> attribute on the <{input}> element
+		provide placeholder text,
+		as does the first <{option}> element of a <{select}> under certain conditions.
+		The '':placeholder-shown'' class thus applies
+		whenever such placeholder text is shown.
+	</div>
 
 <h4 id="the-default-pseudo">
 The Default-option Pseudo-class: '':default''</h4>