diff --git a/cors/cors-safelisted-request-header.any.js b/cors/cors-safelisted-request-header.any.js
index a0a0417d74b3db..05ff22576c5a6d 100644
--- a/cors/cors-safelisted-request-header.any.js
+++ b/cors/cors-safelisted-request-header.any.js
@@ -69,3 +69,12 @@ function safelist(headers, expectPreflight = false) {
 ].forEach(([value, preflight = false]) => {
   safelist({"range": value}, preflight);
 });
+
+[
+  ["abc123"],
+  ["\"", true],
+  ["e5p3n<3k0k0s", true],
+  ["012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678", true],
+].forEach(([value, preflight = false]) => {
+  safelist({"last-event-id": value}, preflight);
+});
diff --git a/eventsource/eventsource-cross-origin-preflight.window.js b/eventsource/eventsource-cross-origin-preflight.window.js
new file mode 100644
index 00000000000000..8af219b7952249
--- /dev/null
+++ b/eventsource/eventsource-cross-origin-preflight.window.js
@@ -0,0 +1,45 @@
+// META: title=EventSource: cross-origin preflight
+// META: script=/common/utils.js
+
+const crossdomain = location.href
+  .replace("://", "://élève.")
+  .replace(/\/[^\/]*$/, "/");
+const origin = location.origin.replace("://", "://xn--lve-6lad.");
+
+[
+  ["safe `last-event-id` (no preflight)", "safe"],
+  ["unsafe `last-event-id` (preflight because too long)", "long"],
+  ["unsafe `last-event-id` (preflight because unsafe characters)", "unsafe"],
+].forEach(([name, fixture]) => {
+  async_test(document.title + " - " + name).step(function () {
+    const uuid = token();
+    const url =
+      crossdomain +
+      "resources/cors-unsafe-last-event-id.py?fixture=" +
+      fixture +
+      "&token=" +
+      uuid;
+
+    const source = new EventSource(url);
+
+    // Make sure to close the EventSource after the test is done.
+    this.add_cleanup(() => source.close());
+
+    // 1. Event will be a `message` with `id` set to a CORS-safe value, then disconnects.
+    source.addEventListener(
+      "message",
+      this.step_func((evt) => assert_equals(evt.data, fixture))
+    );
+
+    // 2. Will emit either `success` or `failure` event. We expect `success`,
+    //    which is the case if `last-event-id` is set to the same value as received above,
+    //    and a preflight request has been sent for the unsafe `last-event-id` headers.
+    source.addEventListener("success", this.step_func_done());
+    source.addEventListener(
+      "failure",
+      this.step_func_done((evt) => {
+        assert_unreached(evt.data);
+      })
+    );
+  });
+});
diff --git a/eventsource/resources/cors-unsafe-last-event-id.py b/eventsource/resources/cors-unsafe-last-event-id.py
new file mode 100644
index 00000000000000..01b5aacefbfd4d
--- /dev/null
+++ b/eventsource/resources/cors-unsafe-last-event-id.py
@@ -0,0 +1,89 @@
+from datetime import datetime
+
+# Beyond the 128-byte limit for `Last-Event-ID`
+long_string = b"a" * 255
+
+# A regular, safe `Last-Event-ID` value
+safe_id_value = b"abc"
+
+# CORS-unsafe request-header byte 0x3C (`<`) in `Last-Event-ID`
+unsafe_id_value = b"e5p3n<3k0k0s"
+
+def main(request, response):
+    origin = request.headers.get(b"Origin")
+    cors_request_headers = request.headers.get(b"Access-Control-Request-Headers")
+
+    # Allow any CORS origin
+    if origin is not None:
+        response.headers.set(b"Access-Control-Allow-Origin", origin)
+
+    # Allow any CORS request headers
+    if cors_request_headers is not None:
+        response.headers.set(b"Access-Control-Allow-Headers", cors_request_headers)
+
+    # Expect a `token` in the query string
+    if b"token" not in request.GET:
+        headers = [(b"Content-Type", b"text/plain")]
+        return 400, headers, b"ERROR: `token` query parameter!"
+
+    # Expect a `fixture` in the query string
+    if b"fixture" not in request.GET:
+        headers = [(b"Content-Type", b"text/plain")]
+        return 400, headers, b"ERROR: `fixture` query parameter!"
+
+    # Prepare state
+    fixture = request.GET.first(b"fixture")
+    token = request.GET.first(b"token")
+    last_event_id = request.headers.get(b"Last-Event-ID", b"")
+    expect_preflight = fixture == b"unsafe" or fixture == b"long"
+
+    # Preflight handling
+    if request.method == u"OPTIONS":
+        # We keep track of the different "tokens" we see, in order to tell whether or not
+        # a client has done a preflight request. If the "stash" does not contain a token,
+        # no preflight request was made.
+        request.server.stash.put(token, cors_request_headers)
+
+        # We can return with an empty body on preflight requests
+        return b""
+
+    # This will be a SSE endpoint
+    response.headers.set(b"Content-Type", b"text/event-stream")
+    response.headers.set(b"Cache-Control", b"no-store")
+
+    # If we do not have a `Last-Event-ID` header, we're on the initial request
+    # Respond with the fixture corresponding to the `fixture` query parameter
+    if last_event_id == b"":
+        if fixture == b"safe":
+            return b"id: " + safe_id_value + b"\nretry: 200\ndata: safe\n\n"
+        if fixture == b"unsafe":
+            return b"id: " + unsafe_id_value + b"\nretry: 200\ndata: unsafe\n\n"
+        if fixture == b"long":
+            return b"id: " + long_string + b"\nretry: 200\ndata: long\n\n"
+        return b"event: failure\ndata: unknown fixture\n\n"
+
+    # If we have a `Last-Event-ID` header, we're on a reconnect.
+    # If fixture is "unsafe", eg requires a preflight, check to see that we got one.
+    preflight_headers = request.server.stash.take(token)
+    saw_preflight = preflight_headers is not None
+    if saw_preflight and not expect_preflight:
+        return b"event: failure\ndata: saw preflight, did not expect one\n\n"
+    elif not saw_preflight and expect_preflight:
+        return b"event: failure\ndata: expected preflight, did not get one\n\n"
+
+    if saw_preflight and preflight_headers.lower() != b"last-event-id":
+        data = b"preflight `access-control-request-headers` was not `last-event-id`"
+        return b"event: failure\ndata: " + data + b"\n\n"
+
+    # Expect to have the same ID in the header as the one we sent.
+    expected = b"<unknown>"
+    if fixture == b"safe":
+        expected = safe_id_value
+    elif fixture == b"unsafe":
+        expected = unsafe_id_value
+    elif fixture == b"long":
+        expected = long_string
+
+    event = last_event_id == expected and b"success" or b"failure"
+    data = b"got " + last_event_id + b", expected " + expected
+    return b"event: " + event + b"\ndata: " + data + b"\n\n"
diff --git a/fetch/api/cors/cors-no-preflight.any.js b/fetch/api/cors/cors-no-preflight.any.js
index 7a0269aae4ec3d..e3063e75a18ad7 100644
--- a/fetch/api/cors/cors-no-preflight.any.js
+++ b/fetch/api/cors/cors-no-preflight.any.js
@@ -39,3 +39,5 @@ corsNoPreflight("Cross domain [GET] [Content-Type: multipart/form-data]", host_i
 corsNoPreflight("Cross domain [GET] [Content-Type: text/plain]", host_info.HTTP_REMOTE_ORIGIN, "GET" , "Content-Type", "text/plain");
 corsNoPreflight("Cross domain [GET] [Content-Type: text/plain;charset=utf-8]", host_info.HTTP_REMOTE_ORIGIN, "GET" , "Content-Type", "text/plain;charset=utf-8");
 corsNoPreflight("Cross domain [GET] [Content-Type: Text/Plain;charset=utf-8]", host_info.HTTP_REMOTE_ORIGIN, "GET" , "Content-Type", "Text/Plain;charset=utf-8");
+corsNoPreflight("Cross domain [GET] [Last-Event-ID: evt-14]", host_info.HTTP_REMOTE_ORIGIN, "GET", "Last-Event-ID", "evt-14");
+corsNoPreflight("Cross domain [GET] [Last-Event-ID: EvT-15]", host_info.HTTP_REMOTE_ORIGIN, "GET", "Last-Event-ID", "EvT-15");
diff --git a/fetch/api/cors/resources/not-cors-safelisted.json b/fetch/api/cors/resources/not-cors-safelisted.json
index 945dc0f93ba4a3..4bee8548b62d75 100644
--- a/fetch/api/cors/resources/not-cors-safelisted.json
+++ b/fetch/api/cors/resources/not-cors-safelisted.json
@@ -8,6 +8,8 @@
   ["content-language", "@"],
   ["content-type", "text/html"],
   ["content-type", "text/plain; long=0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901"],
+  ["last-event-id", "\""],
+  ["last-event-id", "012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678"],
   ["range", "bytes 0-"],
   ["test", "hi"]
 ]
diff --git a/xhr/access-control-basic-cors-safelisted-request-headers.htm b/xhr/access-control-basic-cors-safelisted-request-headers.htm
index 56870493b4ed9d..27ac4a3c9adbda 100644
--- a/xhr/access-control-basic-cors-safelisted-request-headers.htm
+++ b/xhr/access-control-basic-cors-safelisted-request-headers.htm
@@ -17,6 +17,7 @@
       xhr.setRequestHeader("Accept-Language", "ru");
       xhr.setRequestHeader("Content-Language", "ru");
       xhr.setRequestHeader("Content-Type", "text/plain");
+      xhr.setRequestHeader("Last-Event-ID", "abc123");
 
       xhr.send();
 
@@ -24,7 +25,8 @@
           "Accept: *\n" +
           "Accept-Language: ru\n" +
           "Content-Language: ru\n" +
-          "Content-Type: text/plain\n");
+          "Content-Type: text/plain\n" +
+          "Last-Event-ID: abc123\n");
     }, "Request with CORS-safelisted headers");
     </script>
   </body>
diff --git a/xhr/access-control-basic-non-cors-safelisted-last-event-id.htm b/xhr/access-control-basic-non-cors-safelisted-last-event-id.htm
new file mode 100644
index 00000000000000..bb77ee13d58d10
--- /dev/null
+++ b/xhr/access-control-basic-non-cors-safelisted-last-event-id.htm
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <title>Tests cross-origin request with non-CORS-safelisted last-event-id header</title>
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <script src="/common/get-host-info.sub.js"></script>
+  </head>
+  <body>
+    <script type="text/javascript">
+    test(() => {
+      const xhr = new XMLHttpRequest;
+
+      xhr.open("PUT", get_host_info().HTTP_REMOTE_ORIGIN +
+          "/xhr/resources/access-control-basic-put-allow.py", false);
+      xhr.setRequestHeader("Last-Event-ID", "safe-value");
+      xhr.send("PASS: PUT data received");
+
+      assert_equals(xhr.responseText, "PASS: Cross-domain access allowed.\nPASS: PUT data received");
+
+      xhr.open("PUT", get_host_info().HTTP_REMOTE_ORIGIN +
+          "/xhr/resources/access-control-basic-put-allow.py", false);
+      xhr.setRequestHeader("Last-Event-ID", "e5p3n<3k0k0s");
+
+      assert_throws_dom("NetworkError", () => xhr.send("FAIL: PUT data received"));
+      assert_equals(xhr.status, 0, "Cross-domain access was denied in 'send'.");
+    }, "Deny cross-origin request with non-CORS-safelisted last-event-id value");
+    </script>
+  </body>
+</html>
diff --git a/xhr/resources/access-control-basic-cors-safelisted-request-headers.py b/xhr/resources/access-control-basic-cors-safelisted-request-headers.py
index 46523a905a9f70..946e44e4fa68b5 100644
--- a/xhr/resources/access-control-basic-cors-safelisted-request-headers.py
+++ b/xhr/resources/access-control-basic-cors-safelisted-request-headers.py
@@ -11,6 +11,6 @@ def main(request, response):
     response.headers.set(b"Access-Control-Allow-Credentials", b"true")
     response.headers.set(b"Access-Control-Allow-Origin", request.headers.get(b"origin"))
 
-    for header in [b"Accept", b"Accept-Language", b"Content-Language", b"Content-Type"]:
+    for header in [b"Accept", b"Accept-Language", b"Content-Language", b"Content-Type", b"Last-Event-ID"]:
         value = request.headers.get(header)
         response.content += isomorphic_decode(header) + u": " + (isomorphic_decode(value) if value else u"<None>") + u'\n'