[css-pseudo] Add in ::placeholder, per old WG resolution.

Author: fantasai

css-pseudo/Overview.bs

@@ -607,8 +607,13 @@ Security Considerations</h3>
   must prevent pages from being able to read
   the styling of such highlighted segments. 
 
-<h2 id="generated-content">
-Generated Content Pseudo-elements: ''::before'' and ''::after''</h2>
+<h2 id="treelike">
+Tree-Abiding Pseudo-elements</h2>
+
+  These pseudo-elements always fit within the box tree.
+
+<h3 id="generated-content">
+Generated Content Pseudo-elements: ''::before'' and ''::after''</h3>
 
   When their computed 'content' value is not ''content/none'',
   these pseudo-elements generate boxes
@@ -652,7 +657,40 @@ Generated Content Pseudo-elements: ''::before'' and ''::after''</h2>
   (<code>:before</code> and <code>:after</code>)
   for these pseudo-elements.
 
-<h2 id="interactions">Overlapping Pseudo-element Interactions</h2>
+<h3 id="placeholder-pseudo">
+Placeholder Input: the ''::placeholder'' pseudo-element</h3>
+
+  The <dfn>::placeholder</dfn> pseudo-element represents
+  placeholder text in an input field:
+  text that represents the input
+  and provides a hint to the user on how to fill out the form.
+  For example, a date-input field
+  might have the placeholder text “YYYY/MM/DD”
+  to clarify that numeric dates are to be entered in year-month-day order.
+
+  All properties that apply to the ''::first-line'' pseudo-element
+  also apply to the ''::placeholder'' pseudo-element.
+
+  In interactive media, placeholder text is often hidden once the user has entered input;
+  however this is not a requirement, and both the input value and the placeholder text may be visible simultaneously.
+  The exact behavior is UA-defined.
+  Note that in static media (such as print)
+  placeholder text will be present even after the user has entered input.
+
+  Issue: Current bikeshedding, based on implementations and discussions,
+  appears to select from the list
+  <code>::placeholder</code>,
+  <code>::placeholder-text</code>,
+  <code>::input-placeholder</code>.
+  The WG is going with <code>::placeholder</code>.
+  If there are other opinions that provide some kind of argument to the contrary,
+  they should go to <a href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a>.
+
+  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>.
+
+<h2 id="interactions">
+Overlapping Pseudo-element Interactions</h2>
 
   <div class="example" id="overlapping-example">
     Recall that