[scroll-animations-1] Add entry-crossing and exit-crossing timeline ranges

w3c#7973

Author: bramus

scroll-animations-1/Overview.bs

@@ -408,6 +408,28 @@ spec: cssom-view-1; type: dfn;
 			is exiting the [=view progress visibility range=].
 			* 0% is equivalent to 100% of the ''animation-timeline-range/contain'' range.
 			* 100% is equivalent to 100% of the ''animation-timeline-range/cover'' range.
+
+		<dt><dfn>entry-crossing</dfn>
+		<dd>
+			Represents the range during which the [=principal box=] crosses the [=end=]
+			[=border edge=]
+			* 0% progress represents the position at which
+				the [=start=] [=border edge=] of the element’s [=principal box=]
+				coincides with the [=end=] edge of its [=view progress visibility range=].
+			* 100% progress represents the position at which
+				the [=end=] [=border edge=] of the element’s [=principal box=]
+				coincides with the [=end=] edge of its [=view progress visibility range=].
+
+		<dt><dfn>exit-crossing</dfn>
+		<dd>
+			Represents the range during which the [=principal box=] crosses the [=start=]
+			[=border edge=]
+			* 0% progress represents the position at which
+				the [=start=] [=border edge=] of the element’s [=principal box=]
+				coincides with the [=start=] edge of its [=view progress visibility range=].
+			* 100% progress represents the position at which
+				the [=end=] [=border edge=] of the element’s [=principal box=]
+				coincides with the [=start=] edge of its [=view progress visibility range=].
 	</dl>
 
 	ISSUE: Insert diagrams.