Add system check warning for unsafe 'style' attribute

src/django_nh3/apps.py

@@ -6,3 +6,6 @@
 class DjangoNh3AppConfig(AppConfig):
     name = "django_nh3"
     verbose_name = "django-nh3"
+
+    def ready(self):
+        from . import checks  # noqa: F401

src/django_nh3/checks.py

@@ -0,0 +1,33 @@
+from django.conf import settings
+from django.core.checks import Tags, Warning, register
+
+
+@register(Tags.security)
+def check_nh3_settings(app_configs, **kwargs):
+    """
+    Inspects the NH3_ALLOWED_ATTRIBUTES setting to ensure that the 'style'
+    attribute is not allowed, as nh3 does not sanitize CSS content.
+    """
+    errors = []
+    allowed_attributes = getattr(settings, "NH3_ALLOWED_ATTRIBUTES", {})
+
+    found_style = False
+    if isinstance(allowed_attributes, dict):
+        for _tag, attrs in allowed_attributes.items():
+            if "style" in attrs:
+                found_style = True
+                break
+
+    if found_style:
+        errors.append(
+            Warning(
+                "The 'style' attribute is allowed in NH3_ALLOWED_ATTRIBUTES.",
+                hint=(
+                    "Allowing 'style' poses a security risk (XSS) because nh3 "
+                    "does not sanitize CSS content. Ensure you strictly trust "
+                    "the input if this attribute is required."
+                ),
+                id="django_nh3.W001",
+            )
+        )
+    return errors

tests/test_checks.py

@@ -0,0 +1,16 @@
+from django.test import override_settings
+
+from django_nh3.checks import check_nh3_settings
+
+
+@override_settings(NH3_ALLOWED_ATTRIBUTES={"div": {"style"}})
+def test_check_warns_about_style():
+    errors = check_nh3_settings(None)
+    assert len(errors) == 1
+    assert errors[0].id == "django_nh3.W001"
+
+
+@override_settings(NH3_ALLOWED_ATTRIBUTES={"div": {"class", "href"}})
+def test_check_is_silent_when_safe():
+    errors = check_nh3_settings(None)
+    assert len(errors) == 0