[css-view-transitions-1] Clarify SCB for iframe (#10104)

* Fix heading indentation

* Update Overview.bs

* [css-view-transitions-1] Update SCB resolutions

Closes #9786

Author: khushalsagar

css-view-transitions-1/Overview.bs

@@ -768,6 +768,8 @@ 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=].
+
 	<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">
 		<img src="diagrams/phone-browser-snapshot-root.svg" width="200" height="335" alt="The previous diagram, but highlights the area that's the 'snapshot containing block', which includes everything except the top status bar and the bottom bar with the OS back button">
@@ -1213,6 +1215,8 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 ## [=Perform pending transition operations=] ## {#perform-pending-transition-operations-algorithm}
 
 	<div algorithm>
+		<div class=note>This algorithm is invoked as a part of <a href="https://html.spec.whatwg.org/#event-loop-processing-model:perform-pending-transition-operations">update the rendering loop</a> in the html spec.</div>
+
 		To <dfn>perform pending transition operations</dfn> given a {{Document}} |document|, perform the following steps:
 
 		1. If |document|'s [=document/active view transition=] is not null, then:
@@ -1312,6 +1316,8 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 
 		1. Let |document| be |transition|'s [=relevant global object's=] [=associated document=].
 
+		1. If the [=snapshot containing block size=] exceeds an [=implementation-defined=] maximum, then return failure.
+
 		1. Set |transition|'s [=ViewTransition/initial snapshot containing block size=] to the [=snapshot containing block size=].
 
 		1. [=list/For each=] |element| of every [=/element=] that is [=/connected=],