Stub out :user-error pseudo

Author: fantasai

selectors4/Overview.html

@@ -327,7 +327,11 @@ <h2 class="no-num no-toc" id=contents>Table of Contents</h2>
       optionality pseudo-classes ‘<code class=css>:required</code>’ and
       ‘<code class=css>:optional</code>’</a>
 
-     <li><a href="#rw-pseudos"><span class=secno>11.8. </span> The mutability
+     <li><a href="#user-error-pseudos"><span class=secno>11.8. </span> The
+      incorrect-input pseudo-class ‘<code
+      class=css>:user-error</code>’</a>
+
+     <li><a href="#rw-pseudos"><span class=secno>11.9. </span> The mutability
       pseudo-classes ‘<code class=css>:read-only</code>’ and ‘<code
       class=css>:read-write</code>’</a>
     </ul>
@@ -2697,7 +2701,24 @@ <h3 id=opt-pseudos><span class=secno>11.7. </span> The optionality
    submitted. Elements that are not form elements are neither required nor
    optional.
 
-  <h3 id=rw-pseudos><span class=secno>11.8. </span> The mutability
+  <h3 id=user-error-pseudos><span class=secno>11.8. </span> The
+   incorrect-input pseudo-class ’‘<code
+   class=css>:user-error</code>’‘<code class=css></code></h3>
+
+  <p>The <code id=user-error-pseudo>:user-error</code> pseudo-element is
+   intended to represent an input element with incorrect input <em>after</em>
+   the user has interacted with it and attempted to input data into it. The
+   exact rules for when it matches are UA-dependent; however it must match
+   every invalid, out-of-range, or empty-but-required form element between
+   the time the user has attempted to submit the form and before the user has
+   interacted again with the form element, and may match such form elements
+   at other times.
+
+  <p class=issue> What parts of <a
+   href="https://developer.mozilla.org/en-US/docs/CSS/:-moz-ui-invalid">:-moz-ui-invalid
+   description</a> do we want to spec here? Or put as examples?
+
+  <h3 id=rw-pseudos><span class=secno>11.9. </span> The mutability
    pseudo-classes ’‘<code class=css>:read-only</code>’‘<code
    class=css> and </code>’‘<code class=css>:read-write</code>’‘<code
    class=css></code></h3>

selectors4/Overview.src.html

@@ -1949,6 +1949,23 @@ <h3 id="opt-pseudos">
   form it belongs to can be validly submitted. Elements that are not
   form elements are neither required nor optional.
 
+<h3 id="user-error-pseudos">
+The incorrect-input pseudo-class '':user-error''</h3>
+
+  <p>The <code id="user-error-pseudo">:user-error</code> pseudo-element
+  is intended to represent an input element with incorrect input
+  <em>after</em> the user has interacted with it and attempted to input data into it.
+  The exact rules for when it matches are UA-dependent;
+  however it must match every invalid, out-of-range, or empty-but-required form element
+  between the time the user has attempted to submit the form
+  and before the user has interacted again with the form element,
+  and may match such form elements at other times.
+
+  <p class="issue">
+    What parts of <a href="https://developer.mozilla.org/en-US/docs/CSS/:-moz-ui-invalid">:-moz-ui-invalid description</a> do we want to spec here?
+    Or put as examples?
+  </p>
+
 <h3 id="rw-pseudos">
 The mutability pseudo-classes '':read-only'' and '':read-write''</h3>