diff --git a/tests/test_prefs.py b/tests/test_prefs.py
index a4f7857..41c0448 100644
--- a/tests/test_prefs.py
+++ b/tests/test_prefs.py
@@ -1,7 +1,15 @@
+import re
+import sys
+
 import pytest
 
 from invisible_playwright._fpforge import generate_profile
-from invisible_playwright.prefs import translate_profile_to_prefs
+from invisible_playwright.prefs import (
+    _accept_language,
+    _font_metrics_for_platform,
+    _WIN_LIGHT_COLORS,
+    translate_profile_to_prefs,
+)
 
 
 @pytest.mark.unit
@@ -41,3 +49,310 @@ def test_translate_has_stealth_baseline_constants():
     prefs = translate_profile_to_prefs(p)
     assert prefs.get("privacy.resistFingerprinting") is False
     assert "media.peerconnection.enabled" in prefs
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  _accept_language (platform-agnostic)
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_accept_language_with_region():
+    # AL1
+    assert _accept_language("en-US") == "en-US, en"
+
+
+@pytest.mark.unit
+def test_accept_language_no_region():
+    # AL2
+    assert _accept_language("fr") == "fr"
+
+
+@pytest.mark.unit
+def test_accept_language_underscore_normalized():
+    # AL3
+    assert _accept_language("pt_BR") == "pt-BR, pt"
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  _font_metrics_for_platform
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_font_metrics_windows_returns_empty(monkeypatch):
+    # FM2: Windows never applies width-scale factors.
+    monkeypatch.setattr(sys, "platform", "win32")
+    assert _font_metrics_for_platform("Arial|1.0,Verdana|0.9,") == ""
+
+
+@pytest.mark.unit
+def test_font_metrics_empty_input_returns_empty():
+    # FM3: Empty input always returns "" regardless of platform.
+    assert _font_metrics_for_platform("") == ""
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  Platform-specific GPU / MSAA (Windows)
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_gpu_renderer_empty_on_windows(monkeypatch):
+    # PG2
+    monkeypatch.setattr(sys, "platform", "win32")
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["zoom.stealth.webgl.renderer"] == ""
+    assert prefs["zoom.stealth.webgl.vendor"] == ""
+
+
+@pytest.mark.unit
+def test_msaa_pinned_to_4_on_windows(monkeypatch):
+    # PG4: even when profile.webgl.msaa_samples differs, Windows pins to 4.
+    monkeypatch.setattr(sys, "platform", "win32")
+    p = generate_profile(seed=42, pin={"webgl.msaa_samples": 8})
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["zoom.stealth.webgl.msaa"] == 4
+    assert prefs["webgl.msaa-samples"] == 4
+    assert prefs["webgl.msaa-force"] is True
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  Canvas noise skip mask (Windows always uses intel path)
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_canvas_noise_mask_windows_uses_intel_path(monkeypatch):
+    # CN3: on Windows _renderer_lo is hardcoded to "intel" → mask=15.
+    monkeypatch.setattr(sys, "platform", "win32")
+    p = generate_profile(
+        seed=42,
+        pin={"gpu.renderer": "ANGLE (NVIDIA, NVIDIA GeForce RTX 4090 Direct3D11)"},
+    )
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["zoom.stealth.canvas.noise_skip_mask"] == 15
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  WebGL extensions (Windows clears them)
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_webgl_extensions_cleared_on_windows(monkeypatch):
+    # WE2
+    monkeypatch.setattr(sys, "platform", "win32")
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["zoom.stealth.webgl.extensions"] == ""
+    assert prefs["zoom.stealth.webgl2.extensions"] == ""
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  Timezone (platform-agnostic)
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_timezone_set_propagates_to_both_keys():
+    # TZ1
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p, timezone="America/New_York")
+    assert prefs["zoom.stealth.timezone"] == "America/New_York"
+    assert prefs["juggler.timezone.override"] == "America/New_York"
+
+
+@pytest.mark.unit
+def test_timezone_empty_omits_both_keys():
+    # TZ2
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p, timezone="")
+    assert "zoom.stealth.timezone" not in prefs
+    assert "juggler.timezone.override" not in prefs
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  extra_prefs overlay (platform-agnostic)
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_extra_prefs_adds_custom_key():
+    # EP1
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p, extra_prefs={"custom.pref": 42})
+    assert prefs["custom.pref"] == 42
+
+
+@pytest.mark.unit
+def test_extra_prefs_none_value_deletes_key():
+    # EP2
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(
+        p, extra_prefs={"privacy.resistFingerprinting": None}
+    )
+    assert "privacy.resistFingerprinting" not in prefs
+
+
+@pytest.mark.unit
+def test_extra_prefs_overrides_existing_key():
+    # EP3
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p, extra_prefs={"zoom.stealth.seed": 999})
+    assert prefs["zoom.stealth.seed"] == 999
+
+
+@pytest.mark.unit
+def test_extra_prefs_none_is_no_op():
+    # EP4
+    p = generate_profile(seed=42)
+    base = translate_profile_to_prefs(p)
+    with_none = translate_profile_to_prefs(p, extra_prefs=None)
+    assert base == with_none
+
+
+@pytest.mark.unit
+def test_extra_prefs_empty_dict_is_no_op():
+    # EP5
+    p = generate_profile(seed=42)
+    base = translate_profile_to_prefs(p)
+    with_empty = translate_profile_to_prefs(p, extra_prefs={})
+    assert base == with_empty
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  System colors / dark theme (platform-agnostic — palette is Win10)
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_system_colors_present_when_light_theme():
+    # SC1
+    p = generate_profile(seed=42, pin={"dark_theme": False})
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["ui.systemUsesDarkTheme"] == 0
+    # Spot-check a few keys from the Win10 light palette.
+    for key in _WIN_LIGHT_COLORS:
+        assert key in prefs
+        assert prefs[key] == _WIN_LIGHT_COLORS[key]
+
+
+@pytest.mark.unit
+def test_system_colors_absent_when_dark_theme():
+    # SC2
+    p = generate_profile(seed=42, pin={"dark_theme": True})
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["ui.systemUsesDarkTheme"] == 1
+    for key in _WIN_LIGHT_COLORS:
+        assert key not in prefs
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  Locale prefs (platform-agnostic)
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_locale_en_us_accept_languages():
+    # LC1
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p, locale="en-US")
+    assert prefs["intl.accept_languages"] == "en-US, en"
+
+
+@pytest.mark.unit
+def test_locale_underscore_form_normalized():
+    # LC2
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p, locale="de_DE")
+    assert prefs["intl.accept_languages"] == "de-DE, de"
+    assert prefs["general.useragent.locale"] == "de-DE"
+    assert prefs["intl.locale.requested"] == "de-DE"
+
+
+@pytest.mark.unit
+def test_locale_empty_falls_back_to_en_us():
+    # LC3
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p, locale="")
+    assert prefs["intl.accept_languages"] == "en-US, en"
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  Xvfb workarounds (Windows must NOT set Linux-only keys)
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_xvfb_workarounds_absent_on_windows(monkeypatch):
+    # XW2
+    monkeypatch.setattr(sys, "platform", "win32")
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p)
+    assert "gfx.webrender.all" not in prefs
+    assert "gfx.webrender.force-disabled" not in prefs
+    assert "webgl.force-enabled" not in prefs
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  Windows virtual-desktop workarounds
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_virtual_display_workaround_applied_on_windows(monkeypatch):
+    # VD1
+    monkeypatch.setattr(sys, "platform", "win32")
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p, virtual_display=True)
+    assert prefs["security.sandbox.gpu.level"] == 0
+
+
+@pytest.mark.unit
+def test_virtual_display_workaround_absent_when_disabled(monkeypatch):
+    # VD2
+    monkeypatch.setattr(sys, "platform", "win32")
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p, virtual_display=False)
+    assert "security.sandbox.gpu.level" not in prefs
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  Seed-derived LAN IP (platform-agnostic)
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_lan_ip_matches_192_168_pattern():
+    # LI1
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p)
+    ip = prefs["zoom.stealth.webrtc.host_ip"]
+    m = re.match(r"^192\.168\.(\d+)\.(\d+)$", ip)
+    assert m, f"unexpected LAN IP format: {ip!r}"
+    o3, o4 = int(m.group(1)), int(m.group(2))
+    assert 1 <= o3 <= 254
+    assert 1 <= o4 <= 254
+
+
+@pytest.mark.unit
+def test_lan_ip_deterministic_per_seed():
+    # LI2
+    a = translate_profile_to_prefs(generate_profile(seed=42))["zoom.stealth.webrtc.host_ip"]
+    b = translate_profile_to_prefs(generate_profile(seed=42))["zoom.stealth.webrtc.host_ip"]
+    assert a == b
+
+
+@pytest.mark.unit
+def test_lan_ip_seed_zero_has_no_zero_octets():
+    # LI3: code adds +1 so neither dynamic octet should ever be 0.
+    p = generate_profile(seed=0)
+    prefs = translate_profile_to_prefs(p)
+    ip = prefs["zoom.stealth.webrtc.host_ip"]
+    octets = ip.split(".")
+    assert octets[0] == "192"
+    assert octets[1] == "168"
+    assert int(octets[2]) >= 1
+    assert int(octets[3]) >= 1