[cssom-view-1] Editorial scrollIntoView container attribute fixes (#12354)

Author: flackr

cssom-view-1/Overview.bs

@@ -1299,8 +1299,8 @@ The <dfn method for=Element caniuse=scrollintoview>scrollIntoView(<var>arg</var>
     1. Set <var>behavior</var> to the {{ScrollOptions/behavior}} dictionary member of <var>options</var>.
     1. Set <var>block</var> to the {{ScrollIntoViewOptions/block}} dictionary member of <var>options</var>.
     1. Set <var>inline</var> to the {{ScrollIntoViewOptions/inline}} dictionary member of <var>options</var>.
-    1. If the {{ScrollIntoViewOptions/container}} dictionary member of <var>options</var>,
-        set <var>container</var> to this element.
+    1. If the {{ScrollIntoViewOptions/container}} dictionary member of <var>options</var> is "<code>nearest</code>",
+        set <var>container</var> to the element.
 1. Otherwise, if <var>arg</var> is false, then set <var>block</var> to "<code>end</code>".
 1. If the element does not have any associated [=CSS/box=],
     or is not available to user-agent features,
@@ -1545,7 +1545,9 @@ means to run these steps:
                     1. <a for="viewport">Perform a scroll</a> of the <a>viewport</a> to <var>position</var>, with <var>root element</var> as the associated element and <var>behavior</var>
                         as the scroll behavior.
             </dl>
-    1. If <var>container</var> is not null and <var>scrolling box</var> is a [=shadow-including inclusive ancestor=] of <var>container</var>,
+    1. If <var>container</var> is not null and either <var>scrolling box</var>
+        is a [=shadow-including inclusive ancestor=] of <var>container</var> or
+        is a <a>viewport</a> whose <a for="/">document</a> is a [=shadow-including inclusive ancestor=] of <var>container</var>,
         abort the rest of these steps.
 
 </div>