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diff --git a/testing/web-platform/tests/html/browsers/history/joint-session-history/joint-session-history-only-fully-active.html b/testing/web-platform/tests/html/browsers/history/joint-session-history/joint-session-history-only-fully-active.html
new file mode 100644
index 0000000000..c42d160a21
--- /dev/null
+++ b/testing/web-platform/tests/html/browsers/history/joint-session-history/joint-session-history-only-fully-active.html
@@ -0,0 +1,30 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>Do only fully active documents count for session history?</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<body>
+  <iframe id="child"></iframe>
+</body>
+<script>
+  async_test(function(t) {
+    var old_history_len = window.history.length;
+    var child = document.getElementById("child");
+    var timer = window.setInterval(t.step_func(poll), 100);
+    function poll() {
+      if (child.getAttribute("data-child-loaded")) {
+        // Check to see how many entries have been added to the session history.
+        // The spec https://html.spec.whatwg.org/multipage/#joint-session-history
+        // says that only fully active documents are included in the joint session history.
+        // If only fully active documents count, then the only fully active document
+        // is the child, with session length 1, so the joint session length change will be 1.
+        // If all documents count, then the grandchild is reachable via the session history,
+        // and it has session length 1, so the joint session length change will be 2.
+        assert_equals(2, window.history.length - old_history_len);
+        window.clearInterval(timer);
+        t.done();
+      }
+    }
+    child.src = "joint-session-history-child1.html";
+  });
+</script>