diff --git a/flying-saucer-core/src/main/java/org/xhtmlrenderer/css/newmatch/Condition.java b/flying-saucer-core/src/main/java/org/xhtmlrenderer/css/newmatch/Condition.java
index 854bde598..4b960dd88 100644
--- a/flying-saucer-core/src/main/java/org/xhtmlrenderer/css/newmatch/Condition.java
+++ b/flying-saucer-core/src/main/java/org/xhtmlrenderer/css/newmatch/Condition.java
@@ -271,11 +271,13 @@ protected boolean compare(String attrValue, String conditionValue) {
         }
     }
 
-    private static class ClassCondition extends Condition {
-        private final String _paddedClassName;
+    final static class ClassCondition extends Condition {
+        private final String className;
+        private final int classNameLength;
 
         private ClassCondition(String className) {
-            _paddedClassName = " " + className + " ";
+            this.className = className;
+            this.classNameLength = className.length();
         }
 
         @Override
@@ -284,14 +286,27 @@ boolean matches(Node e, AttributeResolver attRes, TreeResolver treeRes) {
                 return false;
             }
             String c = attRes.getClass(e);
-            if (c == null) {
-                return false;
-            }
+            return c != null && containsClassName(c);
+        }
+
+        boolean containsClassName(String classAttribute) {
+            return containsClassName(classAttribute, -1);
+        }
 
+        private boolean containsClassName(String classAttribute, int fromIndex) {
             // This is much faster than calling `split()` and comparing individual values in a loop.
             // NOTE: In jQuery, for example, the attribute value first has whitespace normalized to spaces. But
             // in an XML DOM, space normalization in attributes is supposed to have happened already.
-            return (" " + c + " ").contains(_paddedClassName);
+            int index = classAttribute.indexOf(className, fromIndex);
+            if (index == -1) return false;
+
+            return isWhitespace(classAttribute, index - 1)
+                    && isWhitespace(classAttribute, index + classNameLength)
+                    || containsClassName(classAttribute, index + classNameLength);
+        }
+
+        private boolean isWhitespace(String classAttribute, int index) {
+            return index < 0 || index >= classAttribute.length() || Character.isWhitespace(classAttribute.charAt(index));
         }
     }
 
diff --git a/flying-saucer-core/src/test/java/org/xhtmlrenderer/css/newmatch/ClassConditionTest.java b/flying-saucer-core/src/test/java/org/xhtmlrenderer/css/newmatch/ClassConditionTest.java
new file mode 100644
index 000000000..6160b39c5
--- /dev/null
+++ b/flying-saucer-core/src/test/java/org/xhtmlrenderer/css/newmatch/ClassConditionTest.java
@@ -0,0 +1,58 @@
+package org.xhtmlrenderer.css.newmatch;
+
+import org.junit.jupiter.api.Test;
+import org.xhtmlrenderer.css.newmatch.Condition.ClassCondition;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.xhtmlrenderer.css.newmatch.Condition.createClassCondition;
+
+class ClassConditionTest {
+    private final ClassCondition condition = (ClassCondition) createClassCondition("active");
+
+    @Test
+    void classAttributeEqualsExpectedClassName() {
+        assertThat(condition.containsClassName("active")).isTrue();
+        assertThat(condition.containsClassName("activex")).isFalse();
+    }
+
+    @Test
+    void classAttributeContainsExpectedClassName() {
+        assertThat(condition.containsClassName("foo active bar")).isTrue();
+        assertThat(condition.containsClassName("foo active ")).isTrue();
+        assertThat(condition.containsClassName(" active bar")).isTrue();
+        assertThat(condition.containsClassName(" active ")).isTrue();
+    }
+
+    @Test
+    void classAttributeStartsWithExpectedClassName() {
+        assertThat(condition.containsClassName("active foo bar")).isTrue();
+        assertThat(condition.containsClassName(" active foo bar")).isTrue();
+        assertThat(condition.containsClassName("activex foo bar")).isFalse();
+        assertThat(condition.containsClassName("inactive foo bar")).isFalse();
+    }
+
+    @Test
+    void classAttributeEndsWithExpectedClassName() {
+        assertThat(condition.containsClassName("foo bar active")).isTrue();
+        assertThat(condition.containsClassName("foo bar active ")).isTrue();
+        assertThat(condition.containsClassName("foo bar inactive")).isFalse();
+        assertThat(condition.containsClassName("foo bar activeactive")).isFalse();
+        assertThat(condition.containsClassName("foo bar active-active")).isFalse();
+        assertThat(condition.containsClassName("foo bar active_active")).isFalse();
+        assertThat(condition.containsClassName("foo bar activex")).isFalse();
+    }
+
+    @Test
+    void classAttributeContainsSimilarClassNames() {
+        assertThat(condition.containsClassName("activex active inactive")).isTrue();
+        assertThat(condition.containsClassName("activex _active inactive")).isFalse();
+    }
+
+    @Test
+    void classNotMatches() {
+        assertThat(condition.containsClassName("")).isFalse();
+        assertThat(condition.containsClassName("_active")).isFalse();
+        assertThat(condition.containsClassName("active_")).isFalse();
+        assertThat(condition.containsClassName("act ive")).isFalse();
+    }
+}