[css-view-transitions-1] Make SCB definition in subframes more precise (w3c#10188)

Author: bokand

css-view-transitions-1/Overview.bs

@@ -41,6 +41,7 @@ spec:css22; type:dfn; text:element
 spec:css-break-4; type:dfn; text:fragment
 spec:css-viewport; type:dfn; text:interactive-widget;
 spec:css-display-4; type: dfn; text:invisible;
+spec:css2; type:dfn; text:viewport
 </pre>
 
 <pre class=anchors>
@@ -768,7 +769,7 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 	(and is therefore consistent regardless of root scrollbars or [=interactive-widget|interactive widgets=]).
 	This makes it likely to be consistent for the [=document element=]'s [=captured element/old image=] and [=captured element/new element=].
 
-	For iframes, the [=snapshot containing block=] corresponds to its [=initial containing block=].
+	Within a [=child navigable=], the [=snapshot containing block=] is the union of the navigable's [=viewport=] with any [=scrollbar gutters=].
 
 	<figure>
 		<img src="diagrams/phone-browser.svg" width="200" height="335" alt="A diagram of a phone screen, including a top status bar, a browser URL bar, web-content area with a floating scrollbar, a virtual keyboard, and a bottom bar with an OS back button">