Set keys to traverse accessibility tree. Using focus ring to highlight selected node.

chrome/browser/resources/chromeos/switch_access/switch_access.js

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-console.log("Switch access is enabled");
+var AutomationNode = chrome.automation.AutomationNode;
+
+/**
+ * @constructor
+ */
+var SwitchAccess = function() {
+  console.log("Switch access is enabled");
+
+  // Currently selected node.
+  /** @private {AutomationNode} */
+  this.node_ = null;
+
+  // List of nodes to push to / pop from in case this.node_ is lost.
+  /** @private {!Array<!AutomationNode>} */
+  this.ancestorList_ = [];
+
+  chrome.automation.getDesktop(function(desktop) {
+    this.node_ = desktop;
+    console.log("AutomationNode for desktop is loaded");
+    this.printDetails_();
+
+    document.addEventListener("keyup", function(event) {
+      if (event.key === "1") {
+        console.log("1 = go to previous element");
+        this.moveToPrevious_();
+      } else if (event.key === "2") {
+        console.log("2 = go to next element");
+        this.moveToNext_();
+      } else if (event.key === "3") {
+        console.log("3 = go to child element");
+        this.moveToFirstChild_();
+      } else if (event.key === "4") {
+        console.log("4 = go to parent element");
+        this.moveToParent_();
+      } else if (event.key === "5") {
+        console.log("5 is not yet implemented");
+        console.log("\n");
+      }
+      chrome.accessibilityPrivate.setFocusRing([this.node_.location]);
+    }.bind(this));
+  }.bind(this));
+};
+
+SwitchAccess.prototype = {
+  /**
+   * Move to the previous sibling of this.node_ if it has one.
+   */
+  moveToPrevious_: function() {
+    var previous = this.node_.previousSibling;
+    if (previous) {
+      this.node_ = previous;
+      this.printDetails_();
+    } else {
+      console.log("Node is first of siblings");
+      console.log("\n");
+    }
+  },
+
+  /**
+   * Move to the next sibling of this.node_ if it has one.
+   */
+  moveToNext_: function() {
+    var next = this.node_.nextSibling;
+    if (next) {
+      this.node_ = next;
+      this.printDetails_();
+    } else {
+      console.log("Node is last of siblings");
+      console.log("\n");
+    }
+  },
+
+  /**
+   * Move to the first child of this.node_ if it has one.
+   */
+  moveToFirstChild_: function() {
+    var child = this.node_.firstChild;
+    if (child) {
+      this.ancestorList_.push(this.node_);
+      this.node_ = child;
+      this.printDetails_();
+    } else {
+      console.log("Node has no children");
+      console.log("\n");
+    }
+  },
+
+  /**
+   * Move to the parent of this.node_ if it has one. If it does not have a
+   * parent but it is not the top level root node, then this.node_ lost track of
+   * its neighbors, and we move to an ancestor node.
+   */
+  moveToParent_: function() {
+    var parent = this.node_.parent;
+    if (parent) {
+      this.ancestorList_.pop();
+      this.node_ = parent;
+      this.printDetails_();
+    } else if (this.ancestorList_.length === 0) {
+      console.log("Node has no parent");
+      console.log("\n");
+    } else {
+      console.log(
+          "Node could not find its parent, so moved to recent ancestor");
+      var ancestor = this.ancestorList_.pop();
+      this.node_ = ancestor;
+      this.printDetails_();
+    }
+  },
+
+  /**
+   * Print out details about the currently selected node and the list of
+   * ancestors.
+   *
+   * @private
+   */
+  printDetails_: function() {
+    this.printNode_(this.node_);
+    console.log(this.ancestorList_);
+    console.log("\n");
+  },
+
+  /**
+   * Print out details about a node.
+   *
+   * @param {AutomationNode} node
+   * @private
+   */
+  printNode_: function(node) {
+    if (node) {
+      console.log("Name = " + node.name);
+      console.log("Role = " + node.role);
+      if (!node.parent) {
+        console.log("At index " + node.indexInParent + ", has no parent");
+      } else {
+        var numSiblings = node.parent.children.length;
+        console.log(
+            "At index " + node.indexInParent + ", there are "
+            + numSiblings + " siblings");
+      }
+      console.log("Has " + node.children.length + " children");
+    } else {
+      console.log("Node is null");
+    }
+    console.log(node);
+  }
+};
+
+new SwitchAccess();