[css-pseudo-4] Cross-link to :placeholder-shown, note request to make ::placeholder apply to certain option elements. w3c#2517

Author: fantasai

css-pseudo-4/Overview.bs

@@ -736,6 +736,13 @@ Placeholder Input: the ''::placeholder'' pseudo-element</h3>
   might have the placeholder text “YYYY/MM/DD”
   to clarify that numeric dates are to be entered in year-month-day order.
 
+  Note: There also exists a '':placeholder-shown'' pseudo-<em>class</em>,
+  which applies to (real) elements while they are showing placeholder text,
+  and can be used to style such elements specially.
+  ''::placeholder'' specifically selects
+  a pseudo-element representing the placeholder <em>text</em>,
+  and is thus relatively limited in its abilities.
+
   All properties that apply to the ''::first-line'' pseudo-element
   also apply to the ''::placeholder'' pseudo-element.
 
@@ -748,6 +755,12 @@ Placeholder Input: the ''::placeholder'' pseudo-element</h3>
   Issue: Authors seem to want 'text-align' on the list of supported properties.
   See e.g. <a href="http://css-tricks.com/almanac/selectors/p/placeholder/">comments here</a>.
 
+  Note: It's been requested that ''::placeholder'' also refer to
+  a placeholder which has a corresponding element in the element tree.
+  It's not clear how this should work, but it may be worth doing.
+  See <a href="https://github.com/w3c/csswg-drafts/issues/2517">Issue 2417</a>.
+
+
 <h2 id="interactions">
 Overlapping Pseudo-element Interactions</h2>