diff --git a/configurations/standard.json b/configurations/standard.json
index 782d028..f2fc27f 100644
--- a/configurations/standard.json
+++ b/configurations/standard.json
@@ -156,14 +156,14 @@
         "no-unknown-custom-media": [
             true,
             {
-                "severity": "error",
+                "severity": "warning",
                 "message": "Unknown custom media query \u201c%s\u201d"
             }
         ],
         "no-unknown-custom-properties": [
             true,
             {
-                "severity": "error",
+                "severity": "warning",
                 "message": "Unknown custom property \u201c%s\u201d"
             }
         ],
diff --git a/tools/update_stylelint.py b/tools/update_stylelint.py
index b30b1f4..11c8a9f 100644
--- a/tools/update_stylelint.py
+++ b/tools/update_stylelint.py
@@ -43,6 +43,14 @@ def update_stylelint_rules():
                         break
 
         if 'no-unknown' in rule_name:
+            # Unknown custom properties/media reference values that are
+            # frequently defined where static analysis can't see them (set at
+            # runtime via JS, in inline style attributes, on a page that wasn't
+            # crawled, or built at compile time by PostCSS). A missing one
+            # degrades gracefully to the initial/inherited value rather than
+            # producing invalid CSS, so report it as a warning, not an error.
+            if rule_name in ('no-unknown-custom-properties', 'no-unknown-custom-media'):
+                rule_config["severity"] = "warning"
             rules[rule_name] = [rule_enable, rule_config]
         elif 'no-deprecated' in rule_name:
             rule_config["severity"] = "warning"