diff --git a/tests/Unit/CSSList/AtRuleBlockListTest.php b/tests/Unit/CSSList/AtRuleBlockListTest.php
index 0537f7d1..86411142 100644
--- a/tests/Unit/CSSList/AtRuleBlockListTest.php
+++ b/tests/Unit/CSSList/AtRuleBlockListTest.php
@@ -7,6 +7,7 @@
 use PHPUnit\Framework\TestCase;
 use Sabberworm\CSS\Comment\Commentable;
 use Sabberworm\CSS\CSSList\AtRuleBlockList;
+use Sabberworm\CSS\CSSList\CSSList;
 use Sabberworm\CSS\Renderable;
 
 /**
@@ -17,8 +18,8 @@
 final class AtRuleBlockListTest extends TestCase
 {
     /*
-    * Tests for the implemented interfaces
-    */
+     * Tests for the implemented interfaces and superclasses
+     */
 
     /**
      * @test
@@ -50,6 +51,16 @@ public function implementsCommentable(): void
         self::assertInstanceOf(Commentable::class, $subject);
     }
 
+    /**
+     * @test
+     */
+    public function isCSSList(): void
+    {
+        $subject = new AtRuleBlockList('supports');
+
+        self::assertInstanceOf(CSSList::class, $subject);
+    }
+
     /*
      * not grouped yet
      */
@@ -66,16 +77,6 @@ public function atRuleNameReturnsTypeProvidedToConstructor(): void
         self::assertSame($type, $subject->atRuleName());
     }
 
-    /**
-     * @test
-     */
-    public function getLineNoByDefaultReturnsZero(): void
-    {
-        $subject = new AtRuleBlockList('supports');
-
-        self::assertSame(0, $subject->getLineNo());
-    }
-
     /**
      * @test
      */
diff --git a/tests/Unit/CSSList/CSSListTest.php b/tests/Unit/CSSList/CSSListTest.php
new file mode 100644
index 00000000..69ee4d05
--- /dev/null
+++ b/tests/Unit/CSSList/CSSListTest.php
@@ -0,0 +1,171 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Unit\CSSList;
+
+use PHPUnit\Framework\TestCase;
+use Sabberworm\CSS\Comment\Commentable;
+use Sabberworm\CSS\Renderable;
+use Sabberworm\CSS\RuleSet\DeclarationBlock;
+use Sabberworm\CSS\Tests\Unit\CSSList\Fixtures\ConcreteCSSList;
+
+/**
+ * @covers \Sabberworm\CSS\CSSList\CSSList
+ */
+final class CSSListTest extends TestCase
+{
+    /**
+     * @test
+     */
+    public function implementsRenderable(): void
+    {
+        $subject = new ConcreteCSSList();
+
+        self::assertInstanceOf(Renderable::class, $subject);
+    }
+
+    /**
+     * @test
+     */
+    public function implementsCommentable(): void
+    {
+        $subject = new ConcreteCSSList();
+
+        self::assertInstanceOf(Commentable::class, $subject);
+    }
+
+    /**
+     * @test
+     */
+    public function getLineNoByDefaultReturnsZero(): void
+    {
+        $subject = new ConcreteCSSList();
+
+        self::assertSame(0, $subject->getLineNo());
+    }
+
+    /**
+     * @test
+     */
+    public function getLineNoReturnsLineNumberProvidedToConstructor(): void
+    {
+        $lineNumber = 42;
+
+        $subject = new ConcreteCSSList($lineNumber);
+
+        self::assertSame($lineNumber, $subject->getLineNo());
+    }
+
+    /**
+     * @test
+     */
+    public function getContentsInitiallyReturnsEmptyArray(): void
+    {
+        $subject = new ConcreteCSSList();
+
+        self::assertSame([], $subject->getContents());
+    }
+
+    /**
+     * @return array<string, array{0: list<DeclarationBlock>}>
+     */
+    public static function contentsDataProvider(): array
+    {
+        return [
+            'empty array' => [[]],
+            '1 item' => [[new DeclarationBlock()]],
+            '2 items' => [[new DeclarationBlock(), new DeclarationBlock()]],
+        ];
+    }
+
+    /**
+     * @test
+     *
+     * @param list<DeclarationBlock> $contents
+     *
+     * @dataProvider contentsDataProvider
+     */
+    public function setContentsSetsContents(array $contents): void
+    {
+        $subject = new ConcreteCSSList();
+
+        $subject->setContents($contents);
+
+        self::assertSame($contents, $subject->getContents());
+    }
+
+    /**
+     * @test
+     */
+    public function setContentsReplacesContentsSetInPreviousCall(): void
+    {
+        $subject = new ConcreteCSSList();
+
+        $contents2 = [new DeclarationBlock()];
+
+        $subject->setContents([new DeclarationBlock()]);
+        $subject->setContents($contents2);
+
+        self::assertSame($contents2, $subject->getContents());
+    }
+
+    /**
+     * @test
+     */
+    public function insertBeforeInsertsContentBeforeSibling(): void
+    {
+        $subject = new ConcreteCSSList();
+
+        $bogusOne = new DeclarationBlock();
+        $bogusOne->setSelectors('.bogus-one');
+        $bogusTwo = new DeclarationBlock();
+        $bogusTwo->setSelectors('.bogus-two');
+
+        $item = new DeclarationBlock();
+        $item->setSelectors('.item');
+
+        $sibling = new DeclarationBlock();
+        $sibling->setSelectors('.sibling');
+
+        $subject->setContents([$bogusOne, $sibling, $bogusTwo]);
+
+        self::assertCount(3, $subject->getContents());
+
+        $subject->insertBefore($item, $sibling);
+
+        self::assertCount(4, $subject->getContents());
+        self::assertSame([$bogusOne, $item, $sibling, $bogusTwo], $subject->getContents());
+    }
+
+    /**
+     * @test
+     */
+    public function insertBeforeAppendsIfSiblingNotFound(): void
+    {
+        $subject = new ConcreteCSSList();
+
+        $bogusOne = new DeclarationBlock();
+        $bogusOne->setSelectors('.bogus-one');
+        $bogusTwo = new DeclarationBlock();
+        $bogusTwo->setSelectors('.bogus-two');
+
+        $item = new DeclarationBlock();
+        $item->setSelectors('.item');
+
+        $sibling = new DeclarationBlock();
+        $sibling->setSelectors('.sibling');
+
+        $orphan = new DeclarationBlock();
+        $orphan->setSelectors('.forever-alone');
+
+        $subject->setContents([$bogusOne, $sibling, $bogusTwo]);
+
+        self::assertCount(3, $subject->getContents());
+
+        $subject->insertBefore($item, $orphan);
+
+        self::assertCount(4, $subject->getContents());
+        self::assertSame([$bogusOne, $sibling, $bogusTwo, $item], $subject->getContents());
+    }
+}
diff --git a/tests/Unit/CSSList/DocumentTest.php b/tests/Unit/CSSList/DocumentTest.php
index 3d1b1257..5ac66b5b 100644
--- a/tests/Unit/CSSList/DocumentTest.php
+++ b/tests/Unit/CSSList/DocumentTest.php
@@ -7,6 +7,7 @@
 use PHPUnit\Framework\TestCase;
 use Sabberworm\CSS\Comment\Commentable;
 use Sabberworm\CSS\CSSList\AtRuleBlockList;
+use Sabberworm\CSS\CSSList\CSSList;
 use Sabberworm\CSS\CSSList\Document;
 use Sabberworm\CSS\Property\Charset;
 use Sabberworm\CSS\Property\Import;
@@ -23,8 +24,8 @@
 final class DocumentTest extends TestCase
 {
     /*
-    * Tests for the implemented interfaces
-    */
+     * Tests for the implemented interfaces and superclasses
+     */
 
     /**
      * @test
@@ -42,121 +43,19 @@ public function implementsCommentable(): void
         self::assertInstanceOf(Commentable::class, new Document());
     }
 
-    /*
-     * not grouped yet
-     */
-
-    /**
-     * @test
-     */
-    public function getContentsInitiallyReturnsEmptyArray(): void
-    {
-        $subject = new Document();
-
-        self::assertSame([], $subject->getContents());
-    }
-
-    /**
-     * @return array<string, array{0: list<DeclarationBlock>}>
-     */
-    public static function contentsDataProvider(): array
-    {
-        return [
-            'empty array' => [[]],
-            '1 item' => [[new DeclarationBlock()]],
-            '2 items' => [[new DeclarationBlock(), new DeclarationBlock()]],
-        ];
-    }
-
-    /**
-     * @test
-     *
-     * @param list<DeclarationBlock> $contents
-     *
-     * @dataProvider contentsDataProvider
-     */
-    public function setContentsSetsContents(array $contents): void
-    {
-        $subject = new Document();
-
-        $subject->setContents($contents);
-
-        self::assertSame($contents, $subject->getContents());
-    }
-
-    /**
-     * @test
-     */
-    public function setContentsReplacesContentsSetInPreviousCall(): void
-    {
-        $subject = new Document();
-
-        $contents2 = [new DeclarationBlock()];
-
-        $subject->setContents([new DeclarationBlock()]);
-        $subject->setContents($contents2);
-
-        self::assertSame($contents2, $subject->getContents());
-    }
-
     /**
      * @test
      */
-    public function insertContentBeforeInsertsContentBeforeSibling(): void
+    public function isCSSList(): void
     {
         $subject = new Document();
 
-        $bogusOne = new DeclarationBlock();
-        $bogusOne->setSelectors('.bogus-one');
-        $bogusTwo = new DeclarationBlock();
-        $bogusTwo->setSelectors('.bogus-two');
-
-        $item = new DeclarationBlock();
-        $item->setSelectors('.item');
-
-        $sibling = new DeclarationBlock();
-        $sibling->setSelectors('.sibling');
-
-        $subject->setContents([$bogusOne, $sibling, $bogusTwo]);
-
-        self::assertCount(3, $subject->getContents());
-
-        $subject->insertBefore($item, $sibling);
-
-        self::assertCount(4, $subject->getContents());
-        self::assertSame([$bogusOne, $item, $sibling, $bogusTwo], $subject->getContents());
+        self::assertInstanceOf(CSSList::class, $subject);
     }
 
-    /**
-     * @test
+    /*
+     * not grouped yet
      */
-    public function insertContentBeforeAppendsIfSiblingNotFound(): void
-    {
-        $subject = new Document();
-
-        $bogusOne = new DeclarationBlock();
-        $bogusOne->setSelectors('.bogus-one');
-        $bogusTwo = new DeclarationBlock();
-        $bogusTwo->setSelectors('.bogus-two');
-
-        $item = new DeclarationBlock();
-        $item->setSelectors('.item');
-
-        $sibling = new DeclarationBlock();
-        $sibling->setSelectors('.sibling');
-
-        $orphan = new DeclarationBlock();
-        $orphan->setSelectors('.forever-alone');
-
-        $subject->setContents([$bogusOne, $sibling, $bogusTwo]);
-
-        self::assertCount(3, $subject->getContents());
-
-        $subject->insertBefore($item, $orphan);
-
-        self::assertCount(4, $subject->getContents());
-        self::assertSame([$bogusOne, $sibling, $bogusTwo, $item], $subject->getContents());
-    }
 
     /**
      * @test
diff --git a/tests/Unit/CSSList/Fixtures/ConcreteCSSList.php b/tests/Unit/CSSList/Fixtures/ConcreteCSSList.php
new file mode 100644
index 00000000..250f2a1b
--- /dev/null
+++ b/tests/Unit/CSSList/Fixtures/ConcreteCSSList.php
@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Unit\CSSList\Fixtures;
+
+use Sabberworm\CSS\CSSList\CSSList;
+use Sabberworm\CSS\OutputFormat;
+
+final class ConcreteCSSList extends CSSList
+{
+    public function isRootList(): bool
+    {
+        throw new \BadMethodCallException('Not implemented', 1740395831);
+    }
+
+    public function render(OutputFormat $outputFormat): string
+    {
+        throw new \BadMethodCallException('Not implemented', 1740395836);
+    }
+}
diff --git a/tests/Unit/CSSList/KeyFrameTest.php b/tests/Unit/CSSList/KeyFrameTest.php
index 72ec89a4..a176b3bf 100644
--- a/tests/Unit/CSSList/KeyFrameTest.php
+++ b/tests/Unit/CSSList/KeyFrameTest.php
@@ -6,6 +6,7 @@
 
 use PHPUnit\Framework\TestCase;
 use Sabberworm\CSS\Comment\Commentable;
+use Sabberworm\CSS\CSSList\CSSList;
 use Sabberworm\CSS\CSSList\KeyFrame;
 use Sabberworm\CSS\Property\AtRule;
 use Sabberworm\CSS\Renderable;
@@ -17,8 +18,8 @@
 final class KeyFrameTest extends TestCase
 {
     /*
-    * Tests for the implemented interfaces
-    */
+     * Tests for the implemented interfaces and superclasses
+     */
 
     /**
      * @test
@@ -50,20 +51,20 @@ public function implementsCommentable(): void
         self::assertInstanceOf(Commentable::class, $subject);
     }
 
-    /*
-     * not grouped yet
-     */
-
     /**
      * @test
      */
-    public function getLineNoByDefaultReturnsZero(): void
+    public function isCSSList(): void
     {
         $subject = new KeyFrame();
 
-        self::assertSame(0, $subject->getLineNo());
+        self::assertInstanceOf(CSSList::class, $subject);
     }
 
+    /*
+     * not grouped yet
+     */
+
     /**
      * @test
      */