diff --git a/tests/Unit/CSSList/AtRuleBlockListTest.php b/tests/Unit/CSSList/AtRuleBlockListTest.php
index 6688d079e..0252f7d3f 100644
--- a/tests/Unit/CSSList/AtRuleBlockListTest.php
+++ b/tests/Unit/CSSList/AtRuleBlockListTest.php
@@ -9,6 +9,7 @@
 use Sabberworm\CSS\CSSList\AtRuleBlockList;
 use Sabberworm\CSS\CSSList\CSSBlockList;
 use Sabberworm\CSS\CSSList\CSSList;
+use Sabberworm\CSS\CSSList\CSSListItem;
 use Sabberworm\CSS\Property\AtRule;
 use Sabberworm\CSS\Renderable;
 
@@ -49,6 +50,16 @@ public function implementsCommentable(): void
         self::assertInstanceOf(Commentable::class, $subject);
     }
 
+    /**
+     * @test
+     */
+    public function implementsCSSListItem(): void
+    {
+        $subject = new AtRuleBlockList('supports');
+
+        self::assertInstanceOf(CSSListItem::class, $subject);
+    }
+
     /**
      * @test
      */
diff --git a/tests/Unit/CSSList/CSSListTest.php b/tests/Unit/CSSList/CSSListTest.php
index abe87dd6f..03539533e 100644
--- a/tests/Unit/CSSList/CSSListTest.php
+++ b/tests/Unit/CSSList/CSSListTest.php
@@ -7,6 +7,7 @@
 use PHPUnit\Framework\TestCase;
 use Sabberworm\CSS\Comment\Commentable;
 use Sabberworm\CSS\CSSElement;
+use Sabberworm\CSS\CSSList\CSSListItem;
 use Sabberworm\CSS\Renderable;
 use Sabberworm\CSS\RuleSet\DeclarationBlock;
 use Sabberworm\CSS\Tests\Unit\CSSList\Fixtures\ConcreteCSSList;
@@ -46,6 +47,16 @@ public function implementsCommentable(): void
         self::assertInstanceOf(Commentable::class, $subject);
     }
 
+    /**
+     * @test
+     */
+    public function implementsCSSListItem(): void
+    {
+        $subject = new ConcreteCSSList();
+
+        self::assertInstanceOf(CSSListItem::class, $subject);
+    }
+
     /**
      * @test
      */
diff --git a/tests/Unit/CSSList/KeyFrameTest.php b/tests/Unit/CSSList/KeyFrameTest.php
index 20be302c6..a3a0d43aa 100644
--- a/tests/Unit/CSSList/KeyFrameTest.php
+++ b/tests/Unit/CSSList/KeyFrameTest.php
@@ -7,6 +7,7 @@
 use PHPUnit\Framework\TestCase;
 use Sabberworm\CSS\Comment\Commentable;
 use Sabberworm\CSS\CSSList\CSSList;
+use Sabberworm\CSS\CSSList\CSSListItem;
 use Sabberworm\CSS\CSSList\KeyFrame;
 use Sabberworm\CSS\Property\AtRule;
 use Sabberworm\CSS\Renderable;
@@ -47,6 +48,16 @@ public function implementsCommentable(): void
         self::assertInstanceOf(Commentable::class, $subject);
     }
 
+    /**
+     * @test
+     */
+    public function implementsCSSListItem(): void
+    {
+        $subject = new KeyFrame();
+
+        self::assertInstanceOf(CSSListItem::class, $subject);
+    }
+
     /**
      * @test
      */
diff --git a/tests/Unit/Property/CSSNamespaceTest.php b/tests/Unit/Property/CSSNamespaceTest.php
new file mode 100644
index 000000000..2e4d99222
--- /dev/null
+++ b/tests/Unit/Property/CSSNamespaceTest.php
@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Unit\Property;
+
+use PHPUnit\Framework\TestCase;
+use Sabberworm\CSS\CSSList\CSSListItem;
+use Sabberworm\CSS\Property\CSSNamespace;
+use Sabberworm\CSS\Value\CSSString;
+
+/**
+ * @covers \Sabberworm\CSS\Property\CSSNamespace
+ */
+final class CSSNamespaceTest extends TestCase
+{
+    /**
+     * @var CSSNamespace
+     */
+    private $subject;
+
+    protected function setUp(): void
+    {
+        $this->subject = new CSSNamespace(new CSSString('http://www.w3.org/2000/svg'));
+    }
+
+    /**
+     * @test
+     */
+    public function implementsCSSListItem(): void
+    {
+        self::assertInstanceOf(CSSListItem::class, $this->subject);
+    }
+}
diff --git a/tests/Unit/Property/CharsetTest.php b/tests/Unit/Property/CharsetTest.php
new file mode 100644
index 000000000..e0645f5ef
--- /dev/null
+++ b/tests/Unit/Property/CharsetTest.php
@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Unit\Property;
+
+use PHPUnit\Framework\TestCase;
+use Sabberworm\CSS\CSSList\CSSListItem;
+use Sabberworm\CSS\Property\Charset;
+use Sabberworm\CSS\Value\CSSString;
+
+/**
+ * @covers \Sabberworm\CSS\Property\Charset
+ */
+final class CharsetTest extends TestCase
+{
+    /**
+     * @var Charset
+     */
+    private $subject;
+
+    protected function setUp(): void
+    {
+        $this->subject = new Charset(new CSSString('UTF-8'));
+    }
+
+    /**
+     * @test
+     */
+    public function implementsCSSListItem(): void
+    {
+        self::assertInstanceOf(CSSListItem::class, $this->subject);
+    }
+}
diff --git a/tests/Unit/Property/ImportTest.php b/tests/Unit/Property/ImportTest.php
new file mode 100644
index 000000000..4ec028e3f
--- /dev/null
+++ b/tests/Unit/Property/ImportTest.php
@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Unit\Property;
+
+use PHPUnit\Framework\TestCase;
+use Sabberworm\CSS\CSSList\CSSListItem;
+use Sabberworm\CSS\Property\Import;
+use Sabberworm\CSS\Value\CSSString;
+use Sabberworm\CSS\Value\URL;
+
+/**
+ * @covers \Sabberworm\CSS\Property\Import
+ */
+final class ImportTest extends TestCase
+{
+    /**
+     * @var Import
+     */
+    private $subject;
+
+    protected function setUp(): void
+    {
+        $this->subject = new Import(new URL(new CSSString('https://example.org/')), null);
+    }
+
+    /**
+     * @test
+     */
+    public function implementsCSSListItem(): void
+    {
+        self::assertInstanceOf(CSSListItem::class, $this->subject);
+    }
+}
diff --git a/tests/Unit/RuleSet/AtRuleSetTest.php b/tests/Unit/RuleSet/AtRuleSetTest.php
new file mode 100644
index 000000000..0e3e0c974
--- /dev/null
+++ b/tests/Unit/RuleSet/AtRuleSetTest.php
@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Unit\RuleSet;
+
+use PHPUnit\Framework\TestCase;
+use Sabberworm\CSS\CSSList\CSSListItem;
+use Sabberworm\CSS\RuleSet\AtRuleSet;
+
+/**
+ * @covers \Sabberworm\CSS\RuleSet\AtRuleSet
+ */
+final class AtRuleSetTest extends TestCase
+{
+    /**
+     * @var AtRuleSet
+     */
+    private $subject;
+
+    protected function setUp(): void
+    {
+        $this->subject = new AtRuleSet('supports');
+    }
+
+    /**
+     * @test
+     */
+    public function implementsCSSListItem(): void
+    {
+        self::assertInstanceOf(CSSListItem::class, $this->subject);
+    }
+}
diff --git a/tests/Unit/RuleSet/DeclarationBlockTest.php b/tests/Unit/RuleSet/DeclarationBlockTest.php
new file mode 100644
index 000000000..b473da354
--- /dev/null
+++ b/tests/Unit/RuleSet/DeclarationBlockTest.php
@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Unit\RuleSet;
+
+use PHPUnit\Framework\TestCase;
+use Sabberworm\CSS\CSSList\CSSListItem;
+use Sabberworm\CSS\RuleSet\DeclarationBlock;
+
+/**
+ * @covers \Sabberworm\CSS\RuleSet\DeclarationBlock
+ */
+final class DeclarationBlockTest extends TestCase
+{
+    /**
+     * @var DeclarationBlock
+     */
+    private $subject;
+
+    protected function setUp(): void
+    {
+        $this->subject = new DeclarationBlock();
+    }
+
+    /**
+     * @test
+     */
+    public function implementsCSSListItem(): void
+    {
+        self::assertInstanceOf(CSSListItem::class, $this->subject);
+    }
+}
diff --git a/tests/Unit/RuleSet/RuleSetTest.php b/tests/Unit/RuleSet/RuleSetTest.php
index 7211a77c9..0b1fd8160 100644
--- a/tests/Unit/RuleSet/RuleSetTest.php
+++ b/tests/Unit/RuleSet/RuleSetTest.php
@@ -6,6 +6,7 @@
 
 use PHPUnit\Framework\TestCase;
 use Sabberworm\CSS\CSSElement;
+use Sabberworm\CSS\CSSList\CSSListItem;
 use Sabberworm\CSS\Tests\Unit\RuleSet\Fixtures\ConcreteRuleSet;
 
 /**
@@ -13,13 +14,29 @@
  */
 final class RuleSetTest extends TestCase
 {
+    /**
+     * @var ConcreteRuleSet
+     */
+    private $subject;
+
+    protected function setUp(): void
+    {
+        $this->subject = new ConcreteRuleSet();
+    }
+
     /**
      * @test
      */
     public function implementsCSSElement(): void
     {
-        $subject = new ConcreteRuleSet();
+        self::assertInstanceOf(CSSElement::class, $this->subject);
+    }
 
-        self::assertInstanceOf(CSSElement::class, $subject);
+    /**
+     * @test
+     */
+    public function implementsCSSListItem(): void
+    {
+        self::assertInstanceOf(CSSListItem::class, $this->subject);
     }
 }