--- a/testing/web-platform/tests/intersection-observer/bounding-box.html
+++ b/testing/web-platform/tests/intersection-observer/bounding-box.html
@@ -8,17 +8,17 @@ pre, #log {
   position: absolute;
   top: 0;
   left: 200px;
 }
 #root {
   overflow: visible;
   height: 200px;
   width: 160px;
-  border: 7px solid black;
+  border: 8px solid black;
 }
 #target {
   margin: 10px;
   width: 100px;
   height: 100px;
   padding: 10px;
   background-color: green;
 }
@@ -45,17 +45,40 @@ runTestCycle(function() {
   assert_equals(entries.length, 0, "No initial notifications.");
   runTestCycle(step0, "First rAF.");
 }, "Test that the target's border bounding box is used to calculate intersection.");
 
 function step0() {
   var targetBounds = clientBounds(target);
   target.style.transform = "translateY(195px)";
   runTestCycle(step1, "target.style.transform = 'translateY(195px)'");
-  checkLastEntry(entries, 0, targetBounds.concat(0, 0, 0, 0, 8, 182, 8, 222, false));
+  checkLastEntry(entries, 0, targetBounds.concat(0, 0, 0, 0, 8, 184, 8, 224, false));
 }
 
 function step1() {
   var targetBounds = clientBounds(target);
+  target.style.transform = "translateY(300px)";
+  runTestCycle(step2, "target.style.transform = 'translateY(300px)'");
+  checkLastEntry(entries, 1, targetBounds.concat(26, 146, 221, 224, 8, 184, 8, 224, true));
+}
+
+function step2() {
+  var targetBounds = clientBounds(target);
   target.style.transform = "";
-  checkLastEntry(entries, 1, targetBounds.concat(25, 145, 220, 222, 8, 182, 8, 222, true));
+  target.style.zoom = "2";
+  runTestCycle(step3, "target.style.zoom = 2");
+  checkLastEntry(entries, 2, targetBounds.concat(0, 0, 0, 0, 8, 184, 8, 224, false));
 }
+
+function step3() {
+  var targetBounds = clientBounds(target);
+  var intersectionWidth = (
+      176  // root width including border
+      -8   // root left border
+      -20  // target left margin * target zoom
+  ) / 2;   // convert to target's zoom factor.
+  var intersectionHeight = (216 - 8 - 20) / 2;
+  var intersectionRect = [targetBounds[0], targetBounds[0] + intersectionWidth,
+                          targetBounds[2], targetBounds[2] + intersectionHeight];
+  checkLastEntry(entries, 3, targetBounds.concat(intersectionRect).concat(8, 184, 8, 224, true));
+}
+
 </script>