[TASK] Add assertInstanceOf tests for DeclarationBlock (#1295)

Author: JakeQZ

tests/Unit/RuleSet/DeclarationBlockTest.php

@@ -5,7 +5,9 @@
 namespace Sabberworm\CSS\Tests\Unit\RuleSet;
 
 use PHPUnit\Framework\TestCase;
+use Sabberworm\CSS\CSSElement;
 use Sabberworm\CSS\CSSList\CSSListItem;
+use Sabberworm\CSS\Position\Positionable;
 use Sabberworm\CSS\RuleSet\DeclarationBlock;
 
 /**
@@ -23,11 +25,27 @@ protected function setUp(): void
         $this->subject = new DeclarationBlock();
     }
 
+    /**
+     * @test
+     */
+    public function implementsCSSElement(): void
+    {
+        self::assertInstanceOf(CSSElement::class, $this->subject);
+    }
+
     /**
      * @test
      */
     public function implementsCSSListItem(): void
     {
         self::assertInstanceOf(CSSListItem::class, $this->subject);
     }
+
+    /**
+     * @test
+     */
+    public function implementsPositionable(): void
+    {
+        self::assertInstanceOf(Positionable::class, $this->subject);
+    }
 }