diff --git a/web/core/src/jsMain/kotlin/org/jetbrains/compose/web/css/utils/stylsheet.kt b/web/core/src/jsMain/kotlin/org/jetbrains/compose/web/css/utils/stylsheet.kt
new file mode 100644
index 0000000000..6a6bc42bf9
--- /dev/null
+++ b/web/core/src/jsMain/kotlin/org/jetbrains/compose/web/css/utils/stylsheet.kt
@@ -0,0 +1,14 @@
+/*
+ * Copyright 2020-2021 JetBrains s.r.o. and respective authors and developers.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE.txt file.
+ */
+
+package org.jetbrains.compose.web.css.utils
+
+import org.jetbrains.compose.web.css.StyleSheet
+import org.jetbrains.compose.web.dom.stringPresentation
+
+
+fun StyleSheet.serializeRules(): List<String> {
+    return cssRules.map { it.stringPresentation(indent = " ", delimiter = "") }
+}
\ No newline at end of file
diff --git a/web/core/src/jsTest/kotlin/css/AnimationTests.kt b/web/core/src/jsTest/kotlin/css/AnimationTests.kt
index 73822e0245..629149f066 100644
--- a/web/core/src/jsTest/kotlin/css/AnimationTests.kt
+++ b/web/core/src/jsTest/kotlin/css/AnimationTests.kt
@@ -5,9 +5,9 @@
 package org.jetbrains.compose.web.core.tests.css
 
 import org.jetbrains.compose.web.ExperimentalComposeWebApi
-import org.jetbrains.compose.web.testutils.*
 import org.jetbrains.compose.web.css.*
-import org.jetbrains.compose.web.dom.stringPresentation
+import org.jetbrains.compose.web.css.utils.serializeRules
+import org.jetbrains.compose.web.testutils.runTest
 import org.w3c.dom.HTMLStyleElement
 import org.w3c.dom.css.CSSStyleSheet
 import org.w3c.dom.get
@@ -39,7 +39,7 @@ object AnimationsStyleSheet : StyleSheet() {
 class AnimationTests {
     @Test
     fun animationClassGenerated() = runTest {
-        val generatedRules = AnimationsStyleSheet.cssRules.map { it.stringPresentation(indent = " ", delimiter = "") }
+        val generatedRules = AnimationsStyleSheet.serializeRules()
 
 
         assertContentEquals(
diff --git a/web/core/src/jsTest/kotlin/css/StyleSheetTests.kt b/web/core/src/jsTest/kotlin/css/StyleSheetTests.kt
new file mode 100644
index 0000000000..ac0278b48e
--- /dev/null
+++ b/web/core/src/jsTest/kotlin/css/StyleSheetTests.kt
@@ -0,0 +1,45 @@
+/*
+ * Copyright 2020-2021 JetBrains s.r.o. and respective authors and developers.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE.txt file.
+ */
+
+package org.jetbrains.compose.web.core.tests.css
+
+import org.jetbrains.compose.web.css.Color
+import org.jetbrains.compose.web.css.StyleSheet
+import org.jetbrains.compose.web.css.color
+import org.jetbrains.compose.web.css.utils.serializeRules
+import kotlin.test.Test
+import kotlin.test.assertContentEquals
+
+
+class StyleSheetTests {
+
+    @Test
+    fun extendExistingStyleSheet() {
+        val styleSheet = object : StyleSheet(usePrefix = false) {
+            val someClassName by style {
+                color(Color.red)
+            }
+        }
+
+        val childStyleSheet = object : StyleSheet(styleSheet, usePrefix = false) {
+            val someClassName by style {
+                color(Color.green)
+            }
+        }
+
+        assertContentEquals(
+            listOf(".someClassName { color: red;}", ".someClassName { color: green;}"),
+            styleSheet.serializeRules(),
+            "styleSheet rules"
+        )
+
+        assertContentEquals(
+            listOf(".someClassName { color: red;}", ".someClassName { color: green;}"),
+            childStyleSheet.serializeRules(),
+            "childStyleSheet rules"
+        )
+    }
+
+}
\ No newline at end of file