Index: trunk/resources/data/validator/relation.mapcss
===================================================================
--- trunk/resources/data/validator/relation.mapcss	(revision 18199)
+++ trunk/resources/data/validator/relation.mapcss	(revision 18369)
@@ -10,5 +10,4 @@
 /* see also #9071 */
 relation[type=boundary][!boundary][!disused:boundary],
-relation[type=building][!building],
 relation[type=destination_sign][!destination],
 relation[type=enforcement][!enforcement],
@@ -67,2 +66,8 @@
   throwWarning: tr("way with {0} is part of a {1} route relation", "{0.tag}", "{0.key}");
 }
+
+/* #21285, #21738 */
+relation[type=building][building] >[role=outline] *[building][parent_tag(building) != tag(building)] {
+  throwWarning: tr("{0} of {1} member and {0} relation differ", "{0.key}", "outline");
+  group: tr("suspicious tag combination");
+}