Add a way to find navigable's container

index.bs

@@ -2960,6 +2960,7 @@ To <dfn>await a navigation</dfn> given |navigable|, |request|, |wait condition|,
 browsingContext.Locator = (
    browsingContext.AccessibilityLocator /
    browsingContext.CssLocator /
+   browsingContext.ContainerLocator /
    browsingContext.InnerTextLocator /
    browsingContext.XPathLocator
 )
@@ -2977,6 +2978,10 @@ browsingContext.CssLocator = {
    value: text
 }
 
+browsingContext.ContainerLocator = {
+  type: "container",
+}
+
 browsingContext.InnerTextLocator = {
    type: "innerText",
    value: text,
@@ -3776,6 +3781,18 @@ To <dfn>locate nodes using CSS</dfn> with given |navigable|, |context nodes|,
 
 </div>
 
+<div algorithm="locate the container element">
+To <dfn>locate the container element</dfn> with given |navigable|:
+
+1. Let |returned nodes| be an empty [=/list=].
+
+1. If |navigable|'s [=navigable/container=] is not null,
+   append |navigable|'s [=navigable/container=] to |returned nodes|.
+
+1. Return |returned nodes|.
+
+</div>
+
 <div algorithm="locate nodes using XPath">
 
 To <dfn>locate nodes using XPath</dfn> with given |navigable|, |context nodes|,
@@ -4043,6 +4060,13 @@ The [=remote end steps=] with |session| and |command parameters| are:
          1. Let |result nodes| be [=locate nodes using accessibility attributes=]
             given |context nodes|, |selector|, and |maximum returned node count|.
 
+      <dt>|type| is the string "<code>container</code>"
+      <dd>
+
+         1. Issue: throw errors if extra unused optional params are set?
+
+         1. Let |result nodes| be [=locate the container element=] given |navigable|.
+
 1. Assert: |maximum returned node count| is null or [=list/size=] of |result nodes| is less
    than or equal to |maximum returned node count|.