diff --git a/projects/plugins/jetpack/tests/php/modules/csstidy/test-class.jetpack-csstidy.php b/projects/plugins/jetpack/tests/php/modules/csstidy/test-class.jetpack-csstidy.php
index a30b3bcc4d528..bad30abef9f27 100644
--- a/projects/plugins/jetpack/tests/php/modules/csstidy/test-class.jetpack-csstidy.php
+++ b/projects/plugins/jetpack/tests/php/modules/csstidy/test-class.jetpack-csstidy.php
@@ -28,6 +28,7 @@ public function setUp() {
 		parent::setUp();
 		$this->instance = new csstidy();
 		$this->instance->set_cfg( 'optimise_shorthands', 0 );
+		$this->instance->set_cfg( 'discard_invalid_properties', true );
 	}
 
 	/** Provides values for CSS preserve leading zeros patterns */
@@ -61,4 +62,38 @@ public function test_preserve_leading_zeros( $input, $expected_output, $preserve
 			$this->instance->print->plain()
 		);
 	}
+
+	/** Provides values for CSS custom property patterns */
+	public function custom_property_matches_provider() {
+		// phpcs:ignore Squiz.PHP.CommentedOutCode.Found -- false positive
+		// 'test case description' => [ 'input', 'expected output', 'preserve_css_variables' ].
+		return array(
+			'test_removes_css_var_properties_by_default' => array( 'div {--base-color:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", false ),
+			'test_css_var_properties_preserved'          => array( 'div {--base-color_for_1st-child:red;color:var(--base-color)}', "div {\n--base-color_for_1st-child:red;\ncolor:var(--base-color)\n}", true ),
+			'test_css_var_properties_with_no_alphanum_chars_removed' => array( 'div {--_:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", true ),
+			'test_css_var_properties_ending_in_hyphen_removed' => array( 'div {--base-color-:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", true ),
+			'test_css_var_properties_ending_in_underscore_removed' => array( 'div {--base-color_:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", true ),
+			'test_unknown_properties_removed'            => array( 'div {clowns-nose:red;color:var(--base-color)}', "div {\ncolor:var(--base-color)\n}", true ),
+			'test_invalid_css_properties_removed'        => array( 'div {--$//2343--3423:red;color:var(--$//2343--3423)}', "div {\ncolor:var(--$//2343--3423)\n}", true ),
+			'test_broken_or_dangerous_css_removed'       => array( 'div {xss-trap-be-careful:red;}</style><script>alert(\'Gotcha!\')</script>color:var(--base-color)}', '', true ),
+		);
+	}
+
+	/**
+	 * Test that css variable properties are valid/invalid.
+	 *
+	 * @dataProvider custom_property_matches_provider
+	 *
+	 * @param string $input                  potential CSS custom property.
+	 * @param string $expected_output        what we expect css tidy to output.
+	 * @param bool   $preserve_css_variables the value of preserve_css_variables in csstidy's config.
+	 */
+	public function test_custom_property_patterns( $input, $expected_output, $preserve_css_variables ) {
+		$this->instance->set_cfg( 'preserve_css_variables', $preserve_css_variables );
+		$this->instance->parse( $input );
+		$this->assertEquals(
+			$expected_output,
+			$this->instance->print->plain()
+		);
+	}
 }