diff --git a/packages/style-engine/phpunit/class-wp-style-engine-css-rule-test.php b/packages/style-engine/phpunit/class-wp-style-engine-css-rule-test.php
new file mode 100644
index 00000000000000..9a42ade4acb16f
--- /dev/null
+++ b/packages/style-engine/phpunit/class-wp-style-engine-css-rule-test.php
@@ -0,0 +1,96 @@
+<?php
+/**
+ * Tests the Style Engine CSS Rule class.
+ *
+ * @package    Gutenberg
+ * @subpackage style-engine
+ */
+
+require __DIR__ . '/../class-wp-style-engine-css-rule.php';
+require __DIR__ . '/../class-wp-style-engine-css-declarations.php';
+
+/**
+ * Tests for registering, storing and generating CSS declarations.
+ */
+class WP_Style_Engine_CSS_Rule_Test extends WP_UnitTestCase {
+	/**
+	 * Should set declarations on instantiation.
+	 */
+	public function test_instantiate_with_selector_and_rules() {
+		$selector           = '.law-and-order';
+		$input_declarations = array(
+			'margin-top' => '10px',
+			'font-size'  => '2rem',
+		);
+		$css_declarations   = new WP_Style_Engine_CSS_Declarations( $input_declarations );
+		$css_rule           = new WP_Style_Engine_CSS_Rule( $selector, $css_declarations );
+
+		$this->assertSame( $selector, $css_rule->get_selector() );
+
+		$expected = "$selector {{$css_declarations->get_declarations_string()}}";
+		$this->assertSame( $expected, $css_rule->get_css() );
+	}
+
+	/**
+	 * Test dedupe declaration properties.
+	 */
+	public function test_dedupe_properties_in_rules() {
+		$selector                    = '.taggart';
+		$first_declaration           = array(
+			'font-size' => '2rem',
+		);
+		$overwrite_first_declaration = array(
+			'font-size' => '4px',
+		);
+		$css_rule                    = new WP_Style_Engine_CSS_Rule( $selector, $first_declaration );
+		$css_rule->set_declarations( new WP_Style_Engine_CSS_Declarations( $overwrite_first_declaration ) );
+
+		$expected = '.taggart {font-size: 4px;}';
+		$this->assertSame( $expected, $css_rule->get_css() );
+	}
+
+	/**
+	 * Should set selector and rules on instantiation.
+	 */
+	public function test_set_declarations() {
+		// Declarations using a WP_Style_Engine_CSS_Declarations object.
+		$some_css_declarations = new WP_Style_Engine_CSS_Declarations( array( 'margin-top' => '10px' ) );
+		// Declarations using a property => value array.
+		$some_more_css_declarations = array( 'font-size' => '1rem' );
+		$css_rule                   = new WP_Style_Engine_CSS_Rule( '.hill-street-blues', $some_css_declarations );
+		$css_rule->set_declarations( $some_more_css_declarations );
+
+		$expected = '.hill-street-blues {margin-top: 10px; font-size: 1rem;}';
+		$this->assertSame( $expected, $css_rule->get_css() );
+	}
+
+	/**
+	 * Should set selector and rules on instantiation.
+	 */
+	public function test_set_selector() {
+		$selector = '.taggart';
+		$css_rule = new WP_Style_Engine_CSS_Rule( $selector );
+
+		$this->assertSame( $selector, $css_rule->get_selector() );
+
+		$css_rule->set_selector( '.law-and-order' );
+
+		$this->assertSame( '.law-and-order', $css_rule->get_selector() );
+	}
+
+	/**
+	 * Should set selector and rules on instantiation.
+	 */
+	public function test_get_css() {
+		$selector           = '.chips';
+		$input_declarations = array(
+			'margin-top' => '10px',
+			'font-size'  => '2rem',
+		);
+		$css_declarations   = new WP_Style_Engine_CSS_Declarations( $input_declarations );
+		$css_rule           = new WP_Style_Engine_CSS_Rule( $selector, $css_declarations );
+		$expected           = "$selector {{$css_declarations->get_declarations_string()}}";
+
+		$this->assertSame( $expected, $css_rule->get_css() );
+	}
+}
diff --git a/packages/style-engine/phpunit/class-wp-style-engine-css-rules-store-test.php b/packages/style-engine/phpunit/class-wp-style-engine-css-rules-store-test.php
new file mode 100644
index 00000000000000..390700a9e5ac48
--- /dev/null
+++ b/packages/style-engine/phpunit/class-wp-style-engine-css-rules-store-test.php
@@ -0,0 +1,115 @@
+<?php
+/**
+ * Tests the Style Engine CSS Rules Store class.
+ *
+ * @package    Gutenberg
+ * @subpackage style-engine
+ */
+
+require __DIR__ . '/../class-wp-style-engine-css-rules-store.php';
+require __DIR__ . '/../class-wp-style-engine-css-rule.php';
+require __DIR__ . '/../class-wp-style-engine-css-declarations.php';
+
+/**
+ * Tests for registering, storing and retrieving CSS Rules.
+ */
+class WP_Style_Engine_CSS_Rules_Store_Test extends WP_UnitTestCase {
+	/**
+	 * Should create a new store.
+	 */
+	public function test_create_new_store() {
+		$new_pancakes_store = WP_Style_Engine_CSS_Rules_Store::get_store( 'pancakes-with-strawberries' );
+		$this->assertInstanceOf( 'WP_Style_Engine_CSS_Rules_Store', $new_pancakes_store );
+	}
+
+	/**
+	 * Should return previously created store when the same selector key is passed.
+	 */
+	public function test_get_store() {
+		$new_fish_store = WP_Style_Engine_CSS_Rules_Store::get_store( 'fish-n-chips' );
+		$selector       = '.haddock';
+
+		$new_fish_store->get_rule( $selector )->get_selector();
+		$this->assertEquals( $selector, $new_fish_store->get_rule( $selector )->get_selector() );
+
+		$the_same_fish_store = WP_Style_Engine_CSS_Rules_Store::get_store( 'fish-n-chips' );
+		$this->assertEquals( $selector, $the_same_fish_store->get_rule( $selector )->get_selector() );
+	}
+
+	/**
+	 * Should return a stored rule.
+	 */
+	public function test_get_rule() {
+		$new_pie_store = WP_Style_Engine_CSS_Rules_Store::get_store( 'meat-pie' );
+		$selector      = '.wp-block-sauce a:hover';
+		$store_rule    = $new_pie_store->get_rule( $selector );
+		$expected      = "$selector {}";
+		$this->assertEquals( $expected, $store_rule->get_css() );
+
+		$pie_declarations = array(
+			'color'         => 'brown',
+			'border-color'  => 'yellow',
+			'border-radius' => '10rem',
+		);
+		$css_declarations = new WP_Style_Engine_CSS_Declarations( $pie_declarations );
+		$store_rule->set_declarations( $css_declarations );
+
+		$store_rule = $new_pie_store->get_rule( $selector );
+		$expected   = "$selector {{$css_declarations->get_declarations_string()}}";
+		$this->assertEquals( $expected, $store_rule->get_css() );
+	}
+
+	/**
+	 * Should return all stored rules.
+	 */
+	public function test_get_all_rules() {
+		$new_pizza_store = WP_Style_Engine_CSS_Rules_Store::get_store( 'pizza-with-mozzarella' );
+		$selector        = '.wp-block-anchovies a:hover';
+		$store_rule      = $new_pizza_store->get_rule( $selector );
+		$expected        = array(
+			$selector => $store_rule,
+		);
+
+		$this->assertEquals( $expected, $new_pizza_store->get_all_rules() );
+
+		$pizza_declarations = array(
+			'color'         => 'red',
+			'border-color'  => 'yellow',
+			'border-radius' => '10rem',
+		);
+		$css_declarations   = new WP_Style_Engine_CSS_Declarations( $pizza_declarations );
+		$store_rule->set_declarations( array( $css_declarations ) );
+
+		$expected = array(
+			$selector => $store_rule,
+		);
+		$this->assertEquals( $expected, $new_pizza_store->get_all_rules() );
+
+		$new_pizza_declarations = array(
+			'color'        => 'red',
+			'border-color' => 'red',
+			'font-size'    => '10rem',
+		);
+		$css_declarations       = new WP_Style_Engine_CSS_Declarations( $new_pizza_declarations );
+		$store_rule->set_declarations( array( $css_declarations ) );
+
+		$expected = array(
+			$selector => $store_rule,
+		);
+		$this->assertEquals( $expected, $new_pizza_store->get_all_rules() );
+
+		$new_selector             = '.wp-block-mushroom a:hover';
+		$newer_pizza_declarations = array(
+			'padding' => '100px',
+		);
+		$new_store_rule           = $new_pizza_store->get_rule( $new_selector );
+		$css_declarations         = new WP_Style_Engine_CSS_Declarations( $newer_pizza_declarations );
+		$new_store_rule->set_declarations( array( $css_declarations ) );
+
+		$expected = array(
+			$selector     => $store_rule,
+			$new_selector => $new_store_rule,
+		);
+		$this->assertEquals( $expected, $new_pizza_store->get_all_rules() );
+	}
+}