[css-navigation-1] Change :trigger-link to :nav-source and include submit/form (#14122)

* [css-navigation-1] Change `:trigger-link` to `:nav-source` and include form/submit as well

Resolution:
#11801 (comment)

Closes #11801

* Update css-navigation-1/Overview.bs

Co-authored-by: Bramus <bramus@bram.us>

---------

Co-authored-by: Bramus <bramus@bram.us>

Author: noamr

css-navigation-1/Overview.bs

@@ -596,30 +596,35 @@ by matching the relevant parts of the navigation URL to the link URL.
 NOTE: Some of the design discussion for this feature has been in
 <a href="https://github.com/w3c/csswg-drafts/issues/13163">w3c/csswg-drafts#13163</a>.
 
-<h3 id="trigger-link-pseudo-class">The '':trigger-link'' pseudo-class</h3>
+<h3 id="nav-source-pseudo-class">The '':nav-source'' pseudo-class</h3>
 
-This specification defines a new
-<dfn id="trigger-link-pseudo" selector>'':trigger-link''</dfn>
-that matches link elements that trigger the current navigation.
-
-The '':trigger-link'' pseudo-class matches any element where both:
-* the element matches '':any-link''
-* the [=current navigation state=] is not null, and element is its [=navigation state/source element=].
+This specification defines a <dfn id="nav-source-pseudo" selector>'':nav-source''</dfn> pseudo-class,
+matching the element that triggered the current navigation. It is a CSS reflection of the {{NavigateEvent/sourceElement}} property in the Navigation API.
+The trigger element can be an [^a^], [^area^], [^form^], a [^input/type/submit^] input, or a [^button^].
 
-Issue: should this apply to forms or submit buttons?
+The '':nav-source'' pseudo-class matches any element <var>el</var> where
+the [=current navigation state=] is not null, and <var>el</var> is its [=navigation state/source element=].
 
 <div class="example">
 
-A simple example of a '':trigger-link'' selector is this one,
+A simple example of a '':nav-source'' selector is this one,
 which sets the ''view-transition-name'' for an image thumbnail that is a child of the link that triggers the current navigation.
 
-
-<pre highlight=css>
-:trigger-link .thumb {
+<pre highlight=css>``
+a:nav-source .thumb {
   view-transition-name: active-image;
 }
 </pre>
 
+The '':nav-source'' pseudo-class can also be used for forms and submit buttons:
+
+<pre highlight=css>``
+form:nav-source .submit,
+.submit:nav-source {
+  outline: 3px solid green;
+}
+</pre>
+
 </div>
 
 <h2 id="conditional-navigation-queries">Conditional rules for navigation queries</h2>