Display warnings for unknown at-rules or parsing errors

tests/css/test_errors.py

@@ -17,6 +17,9 @@
      ['ERROR: Relative URI reference without a base URI']),
     ('@import "invalid-protocol://absolute-URL"',
      ['ERROR: Failed to load stylesheet at']),
+    ('test', ['WARNING: Parse error']),
+    ('@test', ['WARNING: Unknown empty rule']),
+    ('@test {}', ['WARNING: Unknown rule']),
 ))
 def test_warnings(source, messages):
     with capture_logs() as logs:

weasyprint/css/__init__.py

@@ -888,9 +888,18 @@ def preprocess_stylesheet(device_media_type, base_url, stylesheet_rules, url_fet
                           ignore_imports=False):
     """Do what can be done early on stylesheet, before being in a document."""
     for rule in stylesheet_rules:
-        if getattr(rule, 'content', None) is None and (
-                rule.type != 'at-rule' or rule.lower_at_keyword != 'import'):
-            continue
+        if getattr(rule, 'content', None) is None:
+            if rule.type == 'error':
+                LOGGER.warning(
+                    "Parse error at %d:%d: %s",
+                    rule.source_line, rule.source_column, rule.message)
+            if rule.type != 'at-rule':
+                continue
+            if rule.lower_at_keyword != 'import':
+                LOGGER.warning(
+                    "Unknown empty rule %s at %d:%d",
+                    rule, rule.source_line, rule.source_column)
+                continue
 
         if rule.type == 'qualified-rule':
             try:
@@ -1096,6 +1105,11 @@ def preprocess_stylesheet(device_media_type, base_url, stylesheet_rules, url_fet
 
             counter_style[name] = counter
 
+        else:
+            LOGGER.warning(
+                "Unknown rule %s at %d:%d",
+                rule, rule.source_line, rule.source_column)
+
 
 def get_all_computed_styles(html, user_stylesheets=None, presentational_hints=False,
                             font_config=None, counter_style=None, page_rules=None,