diff --git a/doc.md b/doc.md
index 5e40da9..42f9ac6 100755
--- a/doc.md
+++ b/doc.md
@@ -732,7 +732,7 @@ To keep track of the amount of transferred data, the XHR Level 2 `upload.onprogr
 
 This test has a couple of complications:
 
-* Some browsers don't have a working `upload.onprogress` event. For this, we use a small blobs instead of a large one and we keep track of progress using the `onload` event. This is referred to as IE11 Workaround (but the same bug was also found in some versions of Edge and Safari)
+* Some browsers don't have a working `upload.onprogress` event. For this, we use a small blobs instead of a large one and we keep track of progress using the `onload` event. This is referred to as IE11 Workaround (the same bug was also found in some versions of Edge and in the PlayStation 4 browser). Affected browsers are detected by checking whether `xhr.upload` is usable, not by sniffing the user agent. Safari is __not__ affected and uses the regular, more accurate upload test
 * When `mpot` is set to `true`, an empty request must first be sent in order to load the CORS headers before the test can start
 
 See the code for more implementation details.
diff --git a/speedtest_worker.js b/speedtest_worker.js
index 79c163b..dc99686 100755
--- a/speedtest_worker.js
+++ b/speedtest_worker.js
@@ -146,22 +146,22 @@ this.addEventListener("message", function(e) {
 				}
 			}
 			if (/Edge.(\d+\.\d+)/i.test(ua)) {
-				//Edge 15 introduced a bug that causes onprogress events to not get fired, we have to use the "small chunks" workaround that reduces accuracy
-				settings.forceIE11Workaround = true;
+				if (typeof s.forceIE11Workaround === "undefined") {
+					//Edge 15 introduced a bug that causes onprogress events to not get fired, we have to use the "small chunks" workaround that reduces accuracy
+					settings.forceIE11Workaround = true;
+				}
 			}
 			if (/PlayStation 4.(\d+\.\d+)/i.test(ua)) {
-				//PS4 browser has the same bug as IE11/Edge
-				settings.forceIE11Workaround = true;
+				if (typeof s.forceIE11Workaround === "undefined") {
+					//PS4 browser has the same bug as IE11/Edge
+					settings.forceIE11Workaround = true;
+				}
 			}
 			if (/Chrome.(\d+)/i.test(ua) && /Android|iPhone|iPad|iPod|Windows Phone/i.test(ua)) {
 				//cheap af
 				//Chrome mobile introduced a limitation somewhere around version 65, we have to limit XHR upload size to 4 megabytes
 				settings.xhr_ul_blob_megabytes = 4;
 			}
-			if (/^((?!chrome|android|crios|fxios).)*safari/i.test(ua)) {
-				//Safari also needs the IE11 workaround but only for the MPOT version
-				settings.forceIE11Workaround = true;
-			}
 			//telemetry_level has to be parsed and not just copied
 			if (typeof s.telemetry_level !== "undefined") settings.telemetry_level = s.telemetry_level === "basic" ? 1 : s.telemetry_level === "full" ? 2 : s.telemetry_level === "debug" ? 3 : 0; // telemetry level
 			//transform test_order to uppercase, just in case
diff --git a/tests/e2e/worker-browser-quirks.spec.js b/tests/e2e/worker-browser-quirks.spec.js
new file mode 100644
index 0000000..b5ff2d6
--- /dev/null
+++ b/tests/e2e/worker-browser-quirks.spec.js
@@ -0,0 +1,91 @@
+const fs = require("node:fs");
+const path = require("node:path");
+const { test, expect } = require("@playwright/test");
+
+const workerSource = fs.readFileSync(path.join(__dirname, "..", "..", "speedtest_worker.js"), "utf8");
+
+const userAgents = {
+  safariMac:
+    "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.4 Safari/605.1.15",
+  safariIos:
+    "Mozilla/5.0 (iPhone; CPU iPhone OS 17_4 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.4 Mobile/15E148 Safari/604.1",
+  chromeIos:
+    "Mozilla/5.0 (iPhone; CPU iPhone OS 17_4 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) CriOS/123.0.6312.52 Mobile/15E148 Safari/604.1",
+  firefoxIos:
+    "Mozilla/5.0 (iPhone; CPU iPhone OS 17_4 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) FxiOS/124.0 Mobile/15E148 Safari/605.1.15",
+  chromeAndroid:
+    "Mozilla/5.0 (Linux; Android 14; Pixel 8) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/123.0.0.0 Mobile Safari/537.36",
+  edge16:
+    "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36 Edge/16.16299",
+  playstation4: "Mozilla/5.0 (PlayStation 4 5.55) AppleWebKit/601.2 (KHTML, like Gecko)"
+};
+
+/*
+  Loads speedtest_worker.js into a page that reports the given user agent, feeds it a "start"
+  command and reports back the settings the worker resolved. The "_" test step is a no-op delay,
+  so the worker settles its settings without performing any network I/O.
+*/
+async function resolveSettings(browser, userAgent, customSettings) {
+  const context = await browser.newContext({ userAgent });
+  try {
+    const page = await context.newPage();
+    await page.goto("about:blank");
+    await page.addScriptTag({ content: workerSource });
+    return await page.evaluate(custom => {
+      window.dispatchEvent(new MessageEvent("message", { data: "start " + JSON.stringify(custom) }));
+      return {
+        userAgent: navigator.userAgent,
+        forceIE11Workaround: settings.forceIE11Workaround,
+        xhr_ul_blob_megabytes: settings.xhr_ul_blob_megabytes
+      };
+    }, Object.assign({ test_order: "_" }, customSettings));
+  } finally {
+    await context.close();
+  }
+}
+
+test.describe("speedtest_worker browser quirks", () => {
+  test("Safari uses the accurate upload test instead of the IE11 workaround", async ({ browser }) => {
+    for (const ua of [userAgents.safariMac, userAgents.safariIos]) {
+      const resolved = await resolveSettings(browser, ua);
+      expect(resolved.userAgent, "context user agent should be applied").toBe(ua);
+      expect(resolved.forceIE11Workaround, `${ua} should not force the IE11 workaround`).toBe(false);
+      expect(resolved.xhr_ul_blob_megabytes).toBe(20);
+    }
+  });
+
+  test("Safari, Chrome and Firefox on iOS resolve to the same upload path", async ({ browser }) => {
+    const safari = await resolveSettings(browser, userAgents.safariIos);
+    const chrome = await resolveSettings(browser, userAgents.chromeIos);
+    const firefox = await resolveSettings(browser, userAgents.firefoxIos);
+    expect(safari.forceIE11Workaround).toBe(chrome.forceIE11Workaround);
+    expect(safari.forceIE11Workaround).toBe(firefox.forceIE11Workaround);
+  });
+
+  test("Safari in MPOT mode also uses the accurate upload test", async ({ browser }) => {
+    const resolved = await resolveSettings(browser, userAgents.safariMac, { mpot: true });
+    expect(resolved.forceIE11Workaround).toBe(false);
+  });
+
+  test("Chrome mobile still caps the upload blob at 4 megabytes", async ({ browser }) => {
+    const resolved = await resolveSettings(browser, userAgents.chromeAndroid);
+    expect(resolved.xhr_ul_blob_megabytes).toBe(4);
+    expect(resolved.forceIE11Workaround).toBe(false);
+  });
+
+  test("Edge and the PlayStation 4 browser keep the IE11 workaround by default", async ({ browser }) => {
+    for (const ua of [userAgents.edge16, userAgents.playstation4]) {
+      const resolved = await resolveSettings(browser, ua);
+      expect(resolved.forceIE11Workaround, `${ua} should keep the IE11 workaround`).toBe(true);
+    }
+  });
+
+  test("an explicitly passed forceIE11Workaround is not overwritten by the quirks", async ({ browser }) => {
+    for (const ua of [userAgents.edge16, userAgents.playstation4]) {
+      const off = await resolveSettings(browser, ua, { forceIE11Workaround: false });
+      expect(off.forceIE11Workaround, `${ua} should honour an explicit false`).toBe(false);
+    }
+    const on = await resolveSettings(browser, userAgents.safariMac, { forceIE11Workaround: true });
+    expect(on.forceIE11Workaround, "an explicit true should still force the workaround").toBe(true);
+  });
+});