Index: trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/MapCSSStyleIndex.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/MapCSSStyleIndex.java	(revision 15990)
+++ trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/MapCSSStyleIndex.java	(revision 15991)
@@ -5,6 +5,8 @@
 
 import java.text.MessageFormat;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.Map;
+import java.util.Set;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
@@ -72,7 +74,14 @@
         // optimization: filter rules for different primitive types
         ruleStream.forEach(rule -> {
-            final Map<String, MapCSSRule> selectorsByBase = rule.selectors.stream()
-                    .collect(Collectors.groupingBy(Selector::getBase,
-                            Collectors.collectingAndThen(Collectors.toList(), selectors -> new MapCSSRule(selectors, rule.declaration))));
+            final Map<String, MapCSSRule> selectorsByBase;
+            final Set<String> bases = rule.selectors.stream().map(Selector::getBase).collect(Collectors.toSet());
+            if (bases.size() == 1) {
+                // reuse rule
+                selectorsByBase = Collections.singletonMap(bases.iterator().next(), rule);
+            } else {
+                selectorsByBase = rule.selectors.stream()
+                        .collect(Collectors.groupingBy(Selector::getBase,
+                                Collectors.collectingAndThen(Collectors.toList(), selectors -> new MapCSSRule(selectors, rule.declaration))));
+            }
             selectorsByBase.forEach((base, optRule) -> {
                 switch (base) {