[css-scroll-snap-2] Add snap event handler attributes for Document and Element (w3c#10215)

* [css-scroll-snap-2] Add snap event handler attributes for Document and Element

This adds the onsnapchanged and onsnapchanging attributes to the
css-scroll-snap-2 specification for elements and Document objects.

Heavily influenced by css-animations extensions to
GlobalInterfaceHandlers[1].

[1] https://drafts.csswg.org/css-animations/#interface-globaleventhandlers

* [css-scroll-snap-2] Add snap event handler attributes

Move to appendix; point to html spec.

* [css-scroll-snap-2] Add snap event handler attributes

Clarify that event handlers are additional attributes.

Author: DavMila

css-scroll-snap-2/Overview.bs

@@ -519,6 +519,8 @@ SnapEvent interface
 	</dl>
 
 	A {{SnapEvent}} should not bubble and should not be cancellable.
+
+
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@@ -601,3 +603,46 @@ Physical Longhands for 'scroll-start-target' {#scroll-start-target-longhands-phy
 	</pre>
 
 	...
+
+Appendix B: Event Handlers {#event-handlers}
+============================================================
+
+ISSUE: This section should be moved to the HTML event handler
+<a href="https://html.spec.whatwg.org/#event-handlers-on-elements,-document-objects,-and-window-objects">specification</a>.
+
+Event handlers on elements, Document objects and Window objects {#event-handlers-on-elements-document-and-window-objects}
+------------------------------------------------------------------------------------------------------
+
+	The following are additional <a>event handlers</a> (and their corresponding <a>event handler event types</a>)
+	that must be supported by all <a>HTML elements</a> as both <a>event handler content attributes</a>
+	and <a>event handler IDL attributes</a>; and that must be supported by all {{Window}} objects and {{Document}} objects, as
+	<a>event handler IDL attributes</a>:
+
+	<table class="data" dfn-type=attribute dfn-for="Document,Element,Window">
+		<tr>
+			<th><a>Event handler</a></th>
+			<th><a>Event handler event type</a></th>
+		<tr>
+			<td><dfn>onsnapchanged</dfn></td>
+			<td>{{snapchanged}}</td>
+		<tr>
+			<td><dfn>onsnapchanging</dfn></td>
+			<td>{{snapchanging}}</td>
+	</table>
+
+
+Extensions to the <code>GlobalEventHandlers</code> Interface Mixin {#interface-globaleventhandlers}
+--------------------------------------------------------------------------------------------------------
+
+	This specification extends the {{GlobalEventHandlers}} interface mixin from
+	HTML to add <a>event handler IDL attributes</a> for {{SnapEvents}} as defined
+	in [[#event-handlers-on-elements-document-and-window-objects]].
+
+	<h4 id="interface-globaleventhandlers-idl">IDL Definition</h4>
+
+	<pre class="idl">
+	partial interface mixin GlobalEventHandlers {
+		attribute EventHandler onsnapchanged;
+		attribute EventHandler onsnapchanging;
+	};
+	</pre>