Add notes for focus, etc. and non-fully active documents

Fixes #5878. Clarifies that the focused element stays the same and is not reset when a document's fully active state is changed, and no events (focus/blur) are fired. Also clarifies that fully active does not impact "being rendered" or viewport intersection.

source

@@ -74554,6 +74554,10 @@ END:VCARD</pre>
   area of the document</dfn>. Which control is so designated changes over time, based on algorithms
   in this specification.</p>
 
+  <p class="note">Even if a document is not <span>fully active</span> and not shown to the user, it
+  can still have a <span>focused area of the document</span>. If a document's <span>fully
+  active</span> state changes, its <span>focused area of the document</span> will stay the same.</p>
+
   <p>The <dfn>currently focused area of a top-level browsing context</dfn> <var>topLevelBC</var> at
   any particular time is the <span>focusable area</span>-or-null returned by this algorithm:</p>
 
@@ -115847,9 +115851,19 @@ console.assert(container.firstChild instanceof SuperP);
   means the element is not <span>being rendered</span>, though this might be overridden by the style
   sheets.</p>
 
+  <p class="note">The <span>fully active</span> state does not affect whether an element is
+  <span>being rendered</span> or not. Even if a document is not <span>fully active</span> and not
+  shown at all to the user, elements within it can still qualify as "being rendered".</p>
+
   <p>An element is said to <dfn>intersect the viewport</dfn> when it is <span>being rendered</span>
   and its associated CSS layout box intersects the <span>viewport</span>.</p>
 
+  <p class="note">Similar to the <span>being rendered</span> state, elements in non-<span>fully
+  active</span> documents can still <span>intersect the viewport</span>. The <span>viewport</span>
+  is not shared between documents and might not always be shown to the user, so an element in a
+  non-<span>fully active</span> document can still intersect the <span>viewport</span> associated
+  with its document.</p>
+
   <p class="note">This specification does not define the precise timing for when the intersection is
   tested, but it is suggested that the timing match that of the Intersection Observer API. <ref
   spec=INTERSECTIONOBSERVER></p>