Centralize checks for <a>, <area>, <form>, and <link>

Require that they are all connected to a document that is fully
active. There is enough difference between implementations that this
appears to be web compatible.

Tests:

* web-platform-tests/wpt#5758
* web-platform-tests/wpt#5759
* web-platform-tests/wpt#5761

Fixes #2615.

source

@@ -13488,19 +13488,10 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
   <p>User agents could also include other information, such as the type of the resource (as given by
   the <code data-x="attr-link-type">type</code> attribute).</p>
 
+  <!-- c.f. <a> and <area>'s similar sections -->
   <p>The <span>activation behavior</span> of <code>link</code> elements that create <span
-  data-x="hyperlink">hyperlinks</span> is to run the following steps:</p>
-
-  <ol><!-- c.f. <a> and <area>'s similar sections -->
-
-   <li><p>If the <code>link</code> element's <span>node document</span> is not <span>fully
-   active</span>, then abort these steps.</p></li>
-
-   <li><p><span data-x="following hyperlinks">Follow the hyperlink</span> created by the
-   <code>link</code> element.</p></li>
-
-  </ol>
-
+  data-x="hyperlink">hyperlinks</span> is to <span data-x="following hyperlinks">follow the
+  hyperlink</span> created by the <code>link</code> element.</p>
 
 
 
@@ -19360,10 +19351,6 @@ interface <dfn>HTMLAnchorElement</dfn> : <span>HTMLElement</span> {
   data-x="hyperlink">hyperlinks</span> is to run the following steps:</p>
 
   <ol><!-- c.f. <link> and <area>'s similar section -->
-   <li><p>If the <code>a</code> element's <span>node document</span> is not <span>fully active</span>, then abort these steps.</p></li>
-   <!-- https://software.hixie.ch/utilities/js/live-dom-viewer/?saved=2033 -->
-   <!-- https://www.hixie.ch/tests/adhoc/html/navigation/iframe/002.html -->
-
    <li><p>If the target of the <code data-x="event-click">click</code> event is an <code>img</code>
    element with an <code data-x="attr-img-ismap">ismap</code> attribute specified, then server-side
    image map processing must be performed, as follows:</p>
@@ -22935,11 +22922,23 @@ interface <dfn>HTMLHyperlinkElementUtils</dfn> {
 
   <h4>Following hyperlinks</h4>
 
+  <p>An element <var>element</var> <dfn>cannot navigate</dfn> if one of the following is true:</p>
+
+  <ul class="brief">
+   <li><var>element</var>'s <span>node document</span> is not <span>fully active</span></li>
+   <li><var>element</var> is not an <code>a</code> element and is not <span>connected</span>.</li>
+  </ul>
+
+  <p class="note">This is also used by <span data-x="concept-form-submit">form submission</span> for
+  the <code>form</code> element. The exception for <code>a</code> elements is for compatibility with
+  web content.</p>
+
   <p>When a user <dfn data-x="following hyperlinks">follows a hyperlink</dfn> created by an element
   <var>subject</var>, optionally with a <var>hyperlink suffix</var>, the user agent must run the
   following steps:</p>
 
   <ol>
+   <li><p>If <var>subject</var> <span>cannot navigate</span>, then return.</p></li>
 
    <li><p>Let <var>replace</var> be false.</p></li>
 
@@ -23038,6 +23037,7 @@ interface <dfn>HTMLHyperlinkElementUtils</dfn> {
   run the following steps:</p>
 
   <ol>
+   <li><p>If <var>subject</var> <span>cannot navigate</span>, then return.</p></li>
 
    <li><p><span data-x="parse a url">Parse</span> the <span>URL</span> given by
    <var>subject</var>'s <code data-x="attr-hyperlink-href">href</code> attribute, relative to
@@ -38616,18 +38616,12 @@ interface <dfn>HTMLAreaElement</dfn> : <span>HTMLElement</span> {
 
   <div w-nodev>
 
-  <p>The <span>activation behavior</span> of <code>area</code> elements is to run the following
-  steps:</p>
-
-  <ol><!-- c.f. <link> and <a>'s similar section -->
-   <li><p>If the <code>area</code> element's <span>node document</span> is not <span>fully
-   active</span>, then abort these steps.</p></li>
-
-   <li><p><span data-x="following hyperlinks">Follow the hyperlink</span> or <span
-   data-x="downloading hyperlinks">download the hyperlink</span> created by the <code>area</code>
-   element, if any, and as determined by the <code data-x="attr-hyperlink-download">download</code>
-   attribute and any expressed user preference.</p></li>
-  </ol>
+  <!-- c.f. <link> and <a>'s similar section -->
+  <p>The <span>activation behavior</span> of <code>area</code> elements is to <span
+  data-x="following hyperlinks">Follow the hyperlink</span> or <span data-x="downloading
+  hyperlinks">download the hyperlink</span> created by the <code>area</code> element, if any, and as
+  determined by the <code data-x="attr-hyperlink-download">download</code> attribute and any
+  expressed user preference.</p>
 
   <p>The IDL attributes <dfn><code data-x="dom-area-alt">alt</code></dfn>, <dfn><code
   data-x="dom-area-coords">coords</code></dfn>, <dfn><code
@@ -55722,19 +55716,11 @@ fur
   following steps:</p>
 
   <ol>
-   <li>
-    <p>If <var>form</var> is not <span>connected</span>, then return.</p>
+   <li><p>If <var>form</var> <span>cannot navigate</span>, then return.</p></li>
 
-    <p class="&#x0058;&#x0058;&#x0058;">This check is currently under discussion, and may be
-    either removed or expanded. See <a href="https://github.com/whatwg/html/issues/2615">issue
-    #2615</a> and <a href="https://github.com/whatwg/html/issues/2708">issue #2708</a>.</p>
-   </li>
-
-   <li><p>Let <var>form document</var> be the <var>form</var>'s
-   <span>node document</span>.</p></li>
+   <li><p>Let <var>form document</var> be <var>form</var>'s <span>node document</span>.</p></li>
 
-   <li id="sandboxSubmitBlocked"><p>If <var>form document</var> has no associated <span
-   data-x="concept-document-bc">browsing context</span>, or its <span>active sandboxing flag
+   <li id="sandboxSubmitBlocked"><p>If <var>form document</var>'s <span>active sandboxing flag
    set</span> has its <span>sandboxed forms browsing context flag</span> set, then return.</p></li>
 
    <li><p>Let <var>form browsing context</var> be the <span data-x="concept-document-bc">browsing