diff --git a/testing/web-platform/tests/fledge/tentative/register-ad-beacon.https.sub.window.js b/testing/web-platform/tests/fledge/tentative/register-ad-beacon.https.sub.window.js
new file mode 100644
index 000000000000..3df6b408a570
--- /dev/null
+++ b/testing/web-platform/tests/fledge/tentative/register-ad-beacon.https.sub.window.js
@@ -0,0 +1,321 @@
+// META: script=/resources/testdriver.js
+// META: script=/common/utils.js
+// META: script=resources/fledge-util.js
+// META: timeout=long
+
+promise_test(async test => {
+  const uuid = generateUuid(test);
+  await runReportTest(
+      test, uuid,
+      // reportResult:
+      null,
+      `registerAdBeacon({beacon: '${createSellerBeaconUrl(uuid)}'});`,
+      // reportWin:
+      null,
+      '',
+      // expectedReportUrls:
+      [`${createSellerBeaconUrl(uuid)}, body: `],
+      // renderUrlOverride:
+      createRenderUrl(
+          uuid,
+          `window.fence.reportEvent({
+              eventType: "beacon",
+              eventData: "",
+              destination: ["seller"]
+          });`)
+  );
+}, 'Seller calls registerAdBeacon().');
+
+promise_test(async test => {
+  const uuid = generateUuid(test);
+  await runReportTest(
+      test, uuid,
+      // reportResult:
+      null,
+      '',
+      // reportWin:
+      null,
+      `registerAdBeacon({beacon: '${createBidderBeaconUrl(uuid)}'});`,
+      // expectedReportUrls:
+      [`${createBidderBeaconUrl(uuid)}, body: `],
+      // renderUrlOverride:
+      createRenderUrl(
+          uuid,
+          `window.fence.reportEvent({
+              eventType: "beacon",
+              eventData: "",
+              destination: ["buyer"]
+          });`)
+  );
+}, 'Buyer calls registerAdBeacon().');
+
+promise_test(async test => {
+  const uuid = generateUuid(test);
+  await runReportTest(
+      test, uuid,
+      // reportResult:
+      null,
+      `registerAdBeacon({beacon: '${createSellerBeaconUrl(uuid)}'});`,
+      // reportWin:
+      null,
+      '',
+      // expectedReportUrls:
+      [`${createSellerBeaconUrl(uuid)}, body: body`],
+      // renderUrlOverride:
+      createRenderUrl(
+          uuid,
+          `window.fence.reportEvent({
+              eventType: "beacon",
+              eventData: "body",
+              destination: ["seller"]
+          });`)
+  );
+}, 'Seller calls registerAdBeacon(), beacon sent with body.');
+
+promise_test(async test => {
+  const uuid = generateUuid(test);
+  await runReportTest(
+      test, uuid,
+      // reportResult:
+      null,
+      '',
+      // reportWin:
+      null,
+      `registerAdBeacon({beacon: '${createBidderBeaconUrl(uuid)}'});`,
+      // expectedReportUrls:
+      [`${createBidderBeaconUrl(uuid)}, body: body`],
+      // renderUrlOverride:
+      createRenderUrl(
+          uuid,
+          `window.fence.reportEvent({
+              eventType: "beacon",
+              eventData: "body",
+              destination: ["buyer"]
+          });`)
+  );
+}, 'Buyer calls registerAdBeacon(), beacon sent with body.');
+
+promise_test(async test => {
+  const uuid = generateUuid(test);
+  await runReportTest(
+      test, uuid,
+      // reportResult:
+      null,
+      `registerAdBeacon({beacon: '${createSellerBeaconUrl(uuid)}'});`,
+      // reportWin:
+      null,
+      '',
+      // expectedReportUrls:
+      [`${createSellerBeaconUrl(uuid)}, body: body1`,
+       `${createSellerBeaconUrl(uuid)}, body: body2`],
+      // renderUrlOverride:
+      createRenderUrl(
+          uuid,
+          `window.fence.reportEvent({
+              eventType: "beacon",
+              eventData: "body1",
+              destination: ["seller"]
+          });
+          window.fence.reportEvent({
+              eventType: "beacon",
+              eventData: "body2",
+              destination: ["seller"]
+          });`)
+  );
+}, 'Seller calls registerAdBeacon(). reportEvent() called twice.');
+
+promise_test(async test => {
+  const uuid = generateUuid(test);
+  await runReportTest(
+      test, uuid,
+      // reportResult:
+      null,
+      '',
+      // reportWin:
+      null,
+      `registerAdBeacon({beacon: '${createBidderBeaconUrl(uuid)}'});`,
+      // expectedReportUrls:
+      [`${createBidderBeaconUrl(uuid)}, body: body1`,
+       `${createBidderBeaconUrl(uuid)}, body: body2`],
+      // renderUrlOverride:
+      createRenderUrl(
+          uuid,
+          `window.fence.reportEvent({
+              eventType: "beacon",
+              eventData: "body1",
+              destination: ["buyer"]
+          });
+          window.fence.reportEvent({
+              eventType: "beacon",
+              eventData: "body2",
+              destination: ["buyer"]
+          });`)
+  );
+}, 'Buyer calls registerAdBeacon(). reportEvent() called twice.');
+
+promise_test(async test => {
+  const uuid = generateUuid(test);
+  await runReportTest(
+      test, uuid,
+      // reportResult:
+      null,
+      `registerAdBeacon({beacon1: '${createSellerBeaconUrl(uuid, '1')}',
+                         beacon2: '${createSellerBeaconUrl(uuid, '2')}'});`,
+      // reportWin:
+      null,
+      '',
+      // expectedReportUrls:
+      [`${createSellerBeaconUrl(uuid, '1')}, body: body1`,
+       `${createSellerBeaconUrl(uuid, '2')}, body: body2`],
+      // renderUrlOverride:
+      createRenderUrl(
+          uuid,
+          `window.fence.reportEvent({
+              eventType: "beacon1",
+              eventData: "body1",
+              destination: ["seller"]
+          });
+          window.fence.reportEvent({
+              eventType: "beacon2",
+              eventData: "body2",
+              destination: ["seller"]
+          });`)
+  );
+}, 'Seller calls registerAdBeacon() with multiple beacons.');
+
+promise_test(async test => {
+  const uuid = generateUuid(test);
+  await runReportTest(
+      test, uuid,
+      // reportResult:
+      null,
+      '',
+      // reportWin:
+      null,
+      `registerAdBeacon({beacon1: '${createBidderBeaconUrl(uuid, '1')}',
+                         beacon2: '${createBidderBeaconUrl(uuid, '2')}'});`,
+      // expectedReportUrls:
+      [`${createBidderBeaconUrl(uuid, '1')}, body: body1`,
+       `${createBidderBeaconUrl(uuid, '2')}, body: body2`],
+      // renderUrlOverride:
+      createRenderUrl(
+          uuid,
+          `window.fence.reportEvent({
+              eventType: "beacon1",
+              eventData: "body1",
+              destination: ["buyer"]
+          });
+          window.fence.reportEvent({
+              eventType: "beacon2",
+              eventData: "body2",
+              destination: ["buyer"]
+          });`)
+  );
+}, 'Buyer calls registerAdBeacon() with multiple beacons.');
+
+promise_test(async test => {
+  const uuid = generateUuid(test);
+  await runReportTest(
+      test, uuid,
+      // reportResult:
+      null,
+      `registerAdBeacon({beacon: '${createSellerBeaconUrl(uuid)}'});`,
+      // reportWin:
+      null,
+      `registerAdBeacon({beacon: '${createBidderBeaconUrl(uuid)}'});`,
+      // expectedReportUrls:
+      [`${createSellerBeaconUrl(uuid)}, body: body`,
+       `${createBidderBeaconUrl(uuid)}, body: body`],
+      // renderUrlOverride:
+      createRenderUrl(
+          uuid,
+          `window.fence.reportEvent({
+              eventType: "beacon",
+              eventData: "body",
+              destination: ["seller","buyer"]
+          });`)
+  );
+}, 'Seller and buyer call registerAdBeacon() with shared reportEvent() call.');
+
+promise_test(async test => {
+  const uuid = generateUuid(test);
+  await runReportTest(
+      test, uuid,
+      // reportResult:
+      null,
+      `registerAdBeacon({beacon: '${createSellerBeaconUrl(uuid)}'});`,
+      // reportWin:
+      null,
+      `registerAdBeacon({beacon: '${createBidderBeaconUrl(uuid)}'});`,
+      // expectedReportUrls:
+      [`${createSellerBeaconUrl(uuid)}, body: body1`,
+       `${createBidderBeaconUrl(uuid)}, body: body2`],
+      // renderUrlOverride:
+      createRenderUrl(
+          uuid,
+          `window.fence.reportEvent({
+            eventType: "beacon",
+            eventData: "body1",
+            destination: ["seller"]
+          });
+          window.fence.reportEvent({
+              eventType: "beacon",
+              eventData: "body2",
+              destination: ["buyer"]
+          });`)
+  );
+}, 'Seller and buyer call registerAdBeacon() with separate reportEvent() calls.');
+
+promise_test(async test => {
+  const uuid = generateUuid(test);
+  await runReportTest(
+      test, uuid,
+      // reportResult:
+      null,
+      // Multiple registerAdBeacon() call should result in an exception,
+      // throwing away all beacons and other types of reports.
+      `sendReportTo('${createSellerReportUrl(uuid)}');
+       registerAdBeacon({beacon: '${createSellerBeaconUrl(uuid)}'});
+       registerAdBeacon({beacon1: '${createSellerBeaconUrl(uuid)}'});`,
+      // reportWin:
+      'sellerSignals === null',
+      `registerAdBeacon({beacon: '${createBidderBeaconUrl(uuid)}'});`,
+      // expectedReportUrls:
+      [`${createBidderBeaconUrl(uuid)}, body: body`],
+      // renderUrlOverride:
+      createRenderUrl(
+          uuid,
+          `window.fence.reportEvent({
+              eventType: "beacon",
+              eventData: "body",
+              destination: ["seller","buyer"]
+          });`)
+  );
+}, 'Seller calls registerAdBeacon() multiple times.');
+
+promise_test(async test => {
+  const uuid = generateUuid(test);
+  await runReportTest(
+      test, uuid,
+      // reportResult:
+      null,
+      `registerAdBeacon({beacon: '${createSellerBeaconUrl(uuid)}'});`,
+      // reportWin:
+      null,
+      // Multiple registerAdBeacon() call should result in an exception,
+      // throwing away all beacons and other types of reports.
+      `sendReportTo('${createBidderReportUrl(uuid)}');
+       registerAdBeacon({beacon: '${createBidderBeaconUrl(uuid)}'});
+       registerAdBeacon({beacon1: '${createBidderBeaconUrl(uuid)}'});`,
+      // expectedReportUrls:
+      [`${createSellerBeaconUrl(uuid)}, body: body`],
+      // renderUrlOverride:
+      createRenderUrl(
+          uuid,
+          `window.fence.reportEvent({
+              eventType: "beacon",
+              eventData: "body",
+              destination: ["seller","buyer"]
+          });`)
+  );
+}, 'Buyer calls registerAdBeacon() multiple times.');
diff --git a/testing/web-platform/tests/fledge/tentative/resources/fenced-frame.sub.py b/testing/web-platform/tests/fledge/tentative/resources/fenced-frame.sub.py
new file mode 100644
index 000000000000..c29bb6fecccf
--- /dev/null
+++ b/testing/web-platform/tests/fledge/tentative/resources/fenced-frame.sub.py
@@ -0,0 +1,19 @@
+# Fenced frame HTML body. Generated by a Python file to avoid having quotes in
+# the injected script escaped, which the test server does to *.html files.
+def main(request, response):
+    response.status = (200, b"OK")
+    response.headers.set(b"Content-Type", b"text/html")
+    response.headers.set(b"Supports-Loading-Mode", b"fenced-frame")
+
+    return """
+        <!DOCTYPE html>
+        <html>
+        <body>
+        <script>
+        {{GET[script]}}
+        </script>
+        </body>
+        </html>
+     """
+
+
diff --git a/testing/web-platform/tests/fledge/tentative/resources/fenced_frame.sub.html b/testing/web-platform/tests/fledge/tentative/resources/fenced_frame.sub.html
deleted file mode 100644
index eaca3f49fd15..000000000000
--- a/testing/web-platform/tests/fledge/tentative/resources/fenced_frame.sub.html
+++ /dev/null
@@ -1,5 +0,0 @@
-<!DOCTYPE html>
-<html>
-<body>
-</body>
-</html>
diff --git a/testing/web-platform/tests/fledge/tentative/resources/fenced_frame.sub.html.headers b/testing/web-platform/tests/fledge/tentative/resources/fenced_frame.sub.html.headers
deleted file mode 100644
index bc74b5851b6c..000000000000
--- a/testing/web-platform/tests/fledge/tentative/resources/fenced_frame.sub.html.headers
+++ /dev/null
@@ -1,2 +0,0 @@
-Content-Type: text/html
-Supports-Loading-Mode: fenced-frame
diff --git a/testing/web-platform/tests/fledge/tentative/resources/fledge-util.js b/testing/web-platform/tests/fledge/tentative/resources/fledge-util.js
index 9f95a59e43a3..6432752e075c 100644
--- a/testing/web-platform/tests/fledge/tentative/resources/fledge-util.js
+++ b/testing/web-platform/tests/fledge/tentative/resources/fledge-util.js
@@ -30,6 +30,17 @@ function createSellerReportUrl(uuid, id = '1') {
                           `seller_report_${id}`);
 }
 
+// Much like above ReportUrl methods, except designed for beacons, which
+// are expected to be POSTs.
+function createBidderBeaconUrl(uuid, id = '1') {
+  return createTrackerUrl(window.location.origin, uuid, `track_post`,
+                          `bidder_beacon_${id}`);
+}
+function createSellerBeaconUrl(uuid, id = '1') {
+  return createTrackerUrl(window.location.origin, uuid, `track_post`,
+                          `seller_beacon_${id}`);
+}
+
 // Generates a UUID and registers a cleanup method with the test fixture to
 // request a URL from the request tracking script that clears all data
 // associated with the generated uuid when requested.
@@ -44,10 +55,13 @@ function generateUuid(test) {
   return uuid;
 }
 
-// Repeatedly requests "request_list" URL until exactly the URLs listed
-// in "expectedRequests" have been observed by the request tracker script (in
+// Repeatedly requests "request_list" URL until exactly the entries in
+// "expectedRequests" have been observed by the request tracker script (in
 // any order, since report URLs are not guaranteed to be sent in any order).
 //
+// Elements of `expectedRequests` should either be URLs, in the case of GET
+// requests, or "<URL>, body: <body>" in the case of POST requests.
+//
 // If any other strings are received from the tracking script, or the tracker
 // script reports an error, fails the test.
 async function waitForObservedRequests(uuid, expectedRequests) {
@@ -129,9 +143,12 @@ function createDecisionScriptUrl(uuid, params = {}) {
 
 // Creates a renderUrl for an ad that runs the passed in "script". "uuid" has
 // no effect, beyond making the URL distinct between tests, and being verified
-// by the decision logic script before accepting a bid.
-function createRenderUrl(uuid) {
-  let url = new URL(`${BASE_URL}resources/fenced_frame.sub.html`);
+// by the decision logic script before accepting a bid. "uuid" is expected to
+// be last.
+function createRenderUrl(uuid, script) {
+  let url = new URL(`${BASE_URL}resources/fenced-frame.sub.py`);
+  if (script)
+    url.searchParams.append('script', script);
   url.searchParams.append('uuid', uuid);
   return url.toString();
 }
@@ -232,11 +249,14 @@ async function runBasicFledgeTestExpectingNoWinner(test, testConfig = {}) {
 // the corresponding reporting method, the report is sent to an error URL.
 // Otherwise, the corresponding 'reportResult' / 'reportWin' values are run.
 //
+// `renderUrlOverride` allows the ad URL of the joined InterestGroup to
+// to be set by the caller.
+//
 // Requesting error report URLs causes waitForObservedRequests() to throw
 // rather than hang.
 async function runReportTest(test, uuid, reportResultSuccessCondition,
                              reportResult, reportWinSuccessCondition, reportWin,
-                             expectedReportUrls) {
+                             expectedReportUrls, renderUrlOverride) {
   if (reportResultSuccessCondition) {
     reportResult = `if (!(${reportResultSuccessCondition})) {
                       sendReportTo('${createSellerReportUrl(uuid, 'error')}');
@@ -265,6 +285,8 @@ async function runReportTest(test, uuid, reportResultSuccessCondition,
 
   let interestGroupOverrides =
       { biddingLogicUrl: createBiddingScriptUrl(biddingScriptUrlParams) };
+  if (renderUrlOverride)
+    interestGroupOverrides.ads = [{renderUrl: renderUrlOverride}]
 
   await joinInterestGroup(test, uuid, interestGroupOverrides);
   await runBasicFledgeAuctionAndNavigate(
diff --git a/testing/web-platform/tests/fledge/tentative/resources/request_tracker.py b/testing/web-platform/tests/fledge/tentative/resources/request_tracker.py
index 5fedd727debf..46da796f3010 100644
--- a/testing/web-platform/tests/fledge/tentative/resources/request_tracker.py
+++ b/testing/web-platform/tests/fledge/tentative/resources/request_tracker.py
@@ -14,7 +14,8 @@ from wptserve.utils import isomorphic_decode, isomorphic_encode
 #
 # Each uuid has a stash entry with a dictionary with two entries:
 #     "trackedRequests" is a list of all observed requested URLs with a
-#         dispatch of "track_get".
+#         dispatch of "track_get" or "track_post". POSTS are in the format
+#         "<url>, body: <body>".
 #     "errors" is a list of an errors that occurred.
 #
 # A dispatch of "request_list" will return the "trackedRequests" dictionary
@@ -63,6 +64,24 @@ def main(request, response):
             stash.put(uuid, server_state)
             return simple_response(request, response, 200, b"OK", b"")
 
+        # Tracks a request that's expected to be a POST.
+        # In addition to the method, check the Content-Type, which is currently
+        # always text/plain, and compare the body against the expected body.
+        if dispatch == b"track_post":
+            contentType = request.headers.get(b"Content-Type", b"missing")
+            if request.method != "POST":
+                server_state["errors"].append(
+                    request.url + " has wrong method: " + request.method)
+            elif not contentType.startswith(b"text/plain"):
+                server_state["errors"].append(
+                    request.url + " has wrong Content-Type: " +
+                    contentType.decode("utf-8"))
+            else:
+                server_state["trackedRequests"].append(
+                    request.url + ", body: " + request.body.decode("utf-8"))
+            stash.put(uuid, server_state)
+            return simple_response(request, response, 200, b"OK", b"")
+
         # Report unrecognized dispatch line.
         server_state["errors"].append(
             request.url + " request with unknown dispatch value received: " +