diff --git a/Userland/Libraries/LibWeb/HTML/NavigatorID.cpp b/Userland/Libraries/LibWeb/HTML/NavigatorID.cpp
index 0a67784628f..919abc9ab12 100644
--- a/Userland/Libraries/LibWeb/HTML/NavigatorID.cpp
+++ b/Userland/Libraries/LibWeb/HTML/NavigatorID.cpp
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2022, Andrew Kaster <akaster@serenityos.org>
+ * Copyright (c) 2024, Jamie Mansfield <jmansfield@cadixdev.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -7,29 +8,49 @@
 #include <AK/ByteString.h>
 #include <LibWeb/HTML/NavigatorID.h>
 #include <LibWeb/Loader/ResourceLoader.h>
+#include <LibWeb/Loader/UserAgent.h>
 
 namespace Web::HTML {
 
 // https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-appversion
 String NavigatorIDMixin::app_version() const
 {
+    auto navigator_compatibility_mode = ResourceLoader::the().navigator_compatibility_mode();
+
     // Must return the appropriate string that starts with "5.0 (", as follows:
 
     // Let trail be the substring of default `User-Agent` value that follows the "Mozilla/" prefix.
     auto user_agent_string = ResourceLoader::the().user_agent();
-
     auto trail = MUST(user_agent_string.substring_from_byte_offset(strlen("Mozilla/"), user_agent_string.bytes().size() - strlen("Mozilla/")));
 
     // If the navigator compatibility mode is Chrome or WebKit
-    // NOTE: We are using Chrome for now. Make sure to update all APIs if you add a toggle for this.
+    if (navigator_compatibility_mode == NavigatorCompatibilityMode::Chrome || navigator_compatibility_mode == NavigatorCompatibilityMode::WebKit) {
+        dbgln("Call to NavigatorIDMixin::app_version, using Chrome or WebKit compatibility mode");
 
-    // Return trail.
-    return trail;
+        // Return trail.
+        return trail;
+    }
 
     // If the navigator compatibility mode is Gecko
-    //    If trail starts with "5.0 (Windows", then return "5.0 (Windows)".
-    //    Otherwise, return the prefix of trail up to but not including the first U+003B (;), concatenated with the
-    //        character U+0029 RIGHT PARENTHESIS. For example, "5.0 (Macintosh)", "5.0 (Android 10)", or "5.0 (X11)".
+    if (navigator_compatibility_mode == NavigatorCompatibilityMode::Gecko) {
+        dbgln("Call to NavigatorIDMixin::app_version, using Gecko compatibility mode");
+
+        // If trail starts with "5.0 (Windows", then return "5.0 (Windows)".
+        if (trail.starts_with_bytes("5.0 (Windows"sv, CaseSensitivity::CaseSensitive))
+            return "5.0 (Windows)"_string;
+
+        // Otherwise, return the prefix of trail up to but not including the first U+003B (;), concatenated with the
+        // character U+0029 RIGHT PARENTHESIS. For example, "5.0 (Macintosh)", "5.0 (Android 10)", or "5.0 (X11)".
+        if (auto index = trail.find_byte_offset(';'); index.has_value()) {
+            StringBuilder output;
+            output.append(MUST(trail.substring_from_byte_offset(0, *index)));
+            output.append(')');
+            return MUST(output.to_string());
+        }
+        return trail;
+    }
+
+    VERIFY_NOT_REACHED();
 }
 
 // https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-platform
@@ -43,6 +64,32 @@ String NavigatorIDMixin::platform() const
     return ResourceLoader::the().platform();
 }
 
+// https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-productsub
+String NavigatorIDMixin::product_sub() const
+{
+    auto navigator_compatibility_mode = ResourceLoader::the().navigator_compatibility_mode();
+
+    // Must return the appropriate string from the following list:
+
+    // If the navigator compatibility mode is Chrome or WebKit
+    if (navigator_compatibility_mode == NavigatorCompatibilityMode::Chrome || navigator_compatibility_mode == NavigatorCompatibilityMode::WebKit) {
+        dbgln("Call to NavigatorIDMixin::product_sub, using Chrome or WebKit compatibility mode");
+
+        // The string "20030107".
+        return "20030107"_string;
+    }
+
+    // If the navigator compatibility mode is Gecko
+    if (navigator_compatibility_mode == NavigatorCompatibilityMode::Gecko) {
+        dbgln("Call to NavigatorIDMixin::product_sub, using Gecko compatibility mode");
+
+        // The string "20100101".
+        return "20100101"_string;
+    }
+
+    VERIFY_NOT_REACHED();
+}
+
 // https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-useragent
 String NavigatorIDMixin::user_agent() const
 {
@@ -50,4 +97,38 @@ String NavigatorIDMixin::user_agent() const
     return ResourceLoader::the().user_agent();
 }
 
+// https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-vendor
+String NavigatorIDMixin::vendor() const
+{
+    auto navigator_compatibility_mode = ResourceLoader::the().navigator_compatibility_mode();
+
+    // Must return the appropriate string from the following list:
+
+    // If the navigator compatibility mode is Chrome
+    if (navigator_compatibility_mode == NavigatorCompatibilityMode::Chrome) {
+        dbgln("Call to NavigatorIDMixin::vendor, using Chrome compatibility mode");
+
+        // The string "Google Inc.".
+        return "Google Inc."_string;
+    }
+
+    // If the navigator compatibility mode is Gecko
+    if (navigator_compatibility_mode == NavigatorCompatibilityMode::Gecko) {
+        dbgln("Call to NavigatorIDMixin::vendor, using Gecko compatibility mode");
+
+        // The empty string.
+        return ""_string;
+    }
+
+    // If the navigator compatibility mode is WebKit
+    if (navigator_compatibility_mode == NavigatorCompatibilityMode::WebKit) {
+        dbgln("Call to NavigatorIDMixin::vendor, using WebKit compatibility mode");
+
+        // The string "Apple Computer, Inc.".
+        return "Apple Computer, Inc."_string;
+    }
+
+    VERIFY_NOT_REACHED();
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/HTML/NavigatorID.h b/Userland/Libraries/LibWeb/HTML/NavigatorID.h
index e1c23fd5b9e..5e68b704bef 100644
--- a/Userland/Libraries/LibWeb/HTML/NavigatorID.h
+++ b/Userland/Libraries/LibWeb/HTML/NavigatorID.h
@@ -32,18 +32,18 @@ public:
     String product() const { return "Gecko"_string; }
 
     // https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-productsub
-    String product_sub() const { return "20030107"_string; } // Compatibility mode "Chrome"
+    String product_sub() const;
 
     // https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-useragent
     String user_agent() const;
 
     // https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-vendor
-    String vendor() const { return "Google Inc."_string; } // Compatibility mode "Chrome"
+    String vendor() const;
 
     // https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-vendorsub
     String vendor_sub() const { return String {}; }
 
-    // NOTE: If the navigator compatibility mode is Gecko, then the user agent must also support the following partial interface:
+    // FIXME: If the navigator compatibility mode is Gecko, then the user agent must also support the following partial interface:
     //       bool taint_enabled()
     //       ByteString oscpu()
 };
diff --git a/Userland/Libraries/LibWeb/Loader/ResourceLoader.cpp b/Userland/Libraries/LibWeb/Loader/ResourceLoader.cpp
index 7c16f36f117..0e902f516eb 100644
--- a/Userland/Libraries/LibWeb/Loader/ResourceLoader.cpp
+++ b/Userland/Libraries/LibWeb/Loader/ResourceLoader.cpp
@@ -20,7 +20,6 @@
 #include <LibWeb/Loader/ProxyMappings.h>
 #include <LibWeb/Loader/Resource.h>
 #include <LibWeb/Loader/ResourceLoader.h>
-#include <LibWeb/Loader/UserAgent.h>
 #include <LibWeb/Platform/EventLoopPlugin.h>
 #include <LibWeb/Platform/Timer.h>
 
@@ -60,6 +59,7 @@ ResourceLoader::ResourceLoader(NonnullRefPtr<ResourceLoaderConnector> connector)
     : m_connector(move(connector))
     , m_user_agent(MUST(String::from_utf8(default_user_agent)))
     , m_platform(MUST(String::from_utf8(default_platform)))
+    , m_navigator_compatibility_mode(default_navigator_compatibility_mode)
 {
 }
 
diff --git a/Userland/Libraries/LibWeb/Loader/ResourceLoader.h b/Userland/Libraries/LibWeb/Loader/ResourceLoader.h
index 2cda8c86ea1..43c23116a67 100644
--- a/Userland/Libraries/LibWeb/Loader/ResourceLoader.h
+++ b/Userland/Libraries/LibWeb/Loader/ResourceLoader.h
@@ -16,6 +16,7 @@
 #include <LibProtocol/Request.h>
 #include <LibURL/URL.h>
 #include <LibWeb/Loader/Resource.h>
+#include <LibWeb/Loader/UserAgent.h>
 #include <LibWeb/Page/Page.h>
 
 namespace Web {
@@ -99,6 +100,9 @@ public:
     String const& platform() const { return m_platform; }
     void set_platform(String platform) { m_platform = move(platform); }
 
+    NavigatorCompatibilityMode navigator_compatibility_mode() { return m_navigator_compatibility_mode; }
+    void set_navigator_compatibility_mode(NavigatorCompatibilityMode mode) { m_navigator_compatibility_mode = mode; }
+
     bool enable_do_not_track() const { return m_enable_do_not_track; }
     void set_enable_do_not_track(bool enable) { m_enable_do_not_track = enable; }
 
@@ -119,6 +123,7 @@ private:
     NonnullRefPtr<ResourceLoaderConnector> m_connector;
     String m_user_agent;
     String m_platform;
+    NavigatorCompatibilityMode m_navigator_compatibility_mode;
     bool m_enable_do_not_track { false };
     Optional<JS::GCPtr<Page>> m_page {};
 };
diff --git a/Userland/Libraries/LibWeb/Loader/UserAgent.h b/Userland/Libraries/LibWeb/Loader/UserAgent.h
index ad75fd8928f..0d9fb896163 100644
--- a/Userland/Libraries/LibWeb/Loader/UserAgent.h
+++ b/Userland/Libraries/LibWeb/Loader/UserAgent.h
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2024, Tim Flynn <trflynn89@serenityos.org>
+ * Copyright (c) 2024, Jamie Mansfield <jmansfield@cadixdev.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -53,10 +54,17 @@ namespace Web {
 #    error Unknown OS
 #endif
 
+enum class NavigatorCompatibilityMode {
+    Chrome,
+    Gecko,
+    WebKit
+};
+
 #define BROWSER_NAME "Ladybird"
 #define BROWSER_VERSION "1.0"
 
 constexpr auto default_user_agent = "Mozilla/5.0 (" OS_STRING "; " CPU_STRING ") " BROWSER_NAME "/" BROWSER_VERSION ""sv;
 constexpr auto default_platform = OS_STRING " " CPU_STRING ""sv;
+constexpr auto default_navigator_compatibility_mode = NavigatorCompatibilityMode::Chrome;
 
 }