[cssom-view-1] Add: run snapshot post-layout state steps #10796 (#11037)

Add "run snapshot post-layout state steps" as resolved in issue #10796.
Let other specifications specify what they snapshot on a
feature-by-feature basis.

cssom-view-1/Overview.bs

@@ -2015,6 +2015,18 @@ Issue: In what order are scrollend events dispatched? Ordered based on scroll st
 </table>
 
 
+Post-Layout State Snapshotting {#post-layout-snapshot}
+======================================================
+
+Some CSS features use post-layout state, like scroll position, as input to the next style and layout update.
+
+When asked to <dfn export>run snapshot post-layout state steps</dfn> for a {{Document}} <var>doc</var>, run these steps:
+
+1. For each CSS feature that needs to snapshot post-layout state, take a snapshot of the relevant state in <var>doc</var>.
+
+The state that is snapshot is defined in other specifications. These steps must not invalidate <var>doc</var> or any other {{Document}}s in such a way that other post-layout snapshotting steps can observe that such snapshotting happened. It follows that the order of which such snapshotting takes place should not matter.
+
+
 Security and Privacy Considerations {#priv-sec}
 ===============================================