diff --git a/testing/web-platform/tests/websockets/handlers/send-backpressure_wsh.py b/testing/web-platform/tests/websockets/handlers/send-backpressure_wsh.py
index fe69b8f9c017..c57b997b7efb 100755
--- a/testing/web-platform/tests/websockets/handlers/send-backpressure_wsh.py
+++ b/testing/web-platform/tests/websockets/handlers/send-backpressure_wsh.py
@@ -2,13 +2,18 @@
 
 import time
 
-# The amount of buffering a WebSocket connection has is not standardised, but
-# it's reasonable to expect that it will not be as large as 8MB.
-MESSAGE_SIZE = 8 * 1024 * 1024
+# The amount of internal buffering a WebSocket connection has is not
+# standardised, and varies depending upon the OS. Setting this number too small
+# will result in false negatives, as the entire message gets buffered. Setting
+# this number too large will result in false positives, when it takes more than
+# 2 seconds to transmit the message anyway. This number was arrived at by
+# trial-and-error.
+MESSAGE_SIZE = 16 * 1024 * 1024
 
 
 def web_socket_do_extra_handshake(request):
-    # Turn off permessage-deflate, otherwise it shrinks our 8MB buffer to 8KB.
+    # Turn off permessage-deflate, otherwise it shrinks our big message to a
+    # tiny message.
     request.ws_extension_processors = []
 
 
diff --git a/testing/web-platform/tests/websockets/stream-tentative/backpressure-receive.any.js b/testing/web-platform/tests/websockets/stream-tentative/backpressure-receive.any.js
index 11f4bd9d3501..b4b090958818 100644
--- a/testing/web-platform/tests/websockets/stream-tentative/backpressure-receive.any.js
+++ b/testing/web-platform/tests/websockets/stream-tentative/backpressure-receive.any.js
@@ -3,11 +3,14 @@
 // META: global=window,worker
 // META: timeout=long
 
-// This test works by using a server WebSocket handler which sends an 8MB
+// Allow for this much timer jitter.
+const JITTER_ALLOWANCE_MS = 200;
+
+// This test works by using a server WebSocket handler which sends a large
 // message, and then sends a second message with the time it measured the first
 // message taking. On the browser side, we wait 2 seconds before reading from
 // the socket. This should ensure it takes at least 2 seconds to finish sending
-// the 8MB message.
+// the large message.
 promise_test(async t => {
   const wss = new WebSocketStream(`${BASEURL}/send-backpressure`);
   const { readable } = await wss.connection;
@@ -16,7 +19,7 @@ promise_test(async t => {
   // Create backpressure for 2 seconds.
   await new Promise(resolve => t.step_timeout(resolve, 2000));
 
-  // Skip the 8MB message.
+  // Skip the large message.
   await reader.read();
 
   // Read the time it took.
@@ -24,6 +27,6 @@ promise_test(async t => {
 
   // A browser can pass this test simply by being slow. This may be a source of
   // flakiness for browsers that do not implement backpressure properly.
-  assert_greater_than_equal(Number(value), 2,
+  assert_greater_than_equal(Number(value), 2 - JITTER_ALLOWANCE_MS / 1000,
                             'data send should have taken at least 2 seconds');
 }, 'backpressure should be applied to received messages');