diff --git a/projects/plugins/jetpack/changelog/update-add-csstidy-config-setting-to-preserve-leading-decimal-zero b/projects/plugins/jetpack/changelog/update-add-csstidy-config-setting-to-preserve-leading-decimal-zero
new file mode 100644
index 000000000000..64a215160d7e
--- /dev/null
+++ b/projects/plugins/jetpack/changelog/update-add-csstidy-config-setting-to-preserve-leading-decimal-zero
@@ -0,0 +1,4 @@
+Significance: minor
+Type: bugfix
+
+Adds a setting to csstidy to tell the optimizer to preserve leading decimal zeros.
diff --git a/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy.php b/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy.php
index ecb5f72931c5..106a770b8bb7 100644
--- a/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy.php
+++ b/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy.php
@@ -280,6 +280,8 @@ function __construct() {
 		$this->settings['timestamp'] = false;
 		$this->settings['template'] = ''; // say that propertie exist
 		$this->set_cfg('template','default'); // call load_template
+		/* Tells csstidy_optimise to keep leading zeros on decimal numbers, e.g., 0.7 */
+		$this->settings['preserve_leading_zeros'] = false;
 		$this->optimise = new csstidy_optimise($this);
 
 		$this->tokens_list = & $GLOBALS['csstidy']['tokens'];
diff --git a/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy_optimise.php b/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy_optimise.php
index a5318a499996..5f4a3843a005 100644
--- a/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy_optimise.php
+++ b/projects/plugins/jetpack/modules/custom-css/csstidy/class.csstidy_optimise.php
@@ -401,10 +401,13 @@ function AnalyseCssNumber($string) {
 
 		$return[0] = (float) $string;
 		if (abs($return[0]) > 0 && abs($return[0]) < 1) {
-			if ($return[0] < 0) {
-				$return[0] = '-' . ltrim(substr($return[0], 1), '0');
-			} else {
-				$return[0] = ltrim($return[0], '0');
+			// Removes the initial `0` from a decimal number, e.g., `0.7 => .7` or `-0.666 => -.666`.
+			if ( ! $this->parser->get_cfg( 'preserve_leading_zeros' ) ) {
+				if ( $return[0] < 0 ) {
+					$return[0] = '-' . ltrim( substr( $return[0], 1 ), '0' );
+				} else {
+					$return[0] = ltrim( $return[0], '0' );
+				}
 			}
 		}
 
diff --git a/projects/plugins/jetpack/modules/custom-css/custom-css.php b/projects/plugins/jetpack/modules/custom-css/custom-css.php
index d821c77fef3d..a62e2fe703f8 100644
--- a/projects/plugins/jetpack/modules/custom-css/custom-css.php
+++ b/projects/plugins/jetpack/modules/custom-css/custom-css.php
@@ -1623,9 +1623,10 @@ static function filter_attr( $css, $element = 'div' ) {
 		$csstidy->set_cfg( 'remove_last_;', false );
 		$csstidy->set_cfg( 'css_level', 'CSS3.0' );
 
-		// Turn off css shorthands when in block editor context as it breaks block validation.
+		// Turn off css shorthands and leading zero removal when in block editor context as it breaks block validation.
 		if ( true === isset( $_REQUEST['_gutenberg_nonce'] ) && wp_verify_nonce( $_REQUEST['_gutenberg_nonce'], 'gutenberg_request' ) ) { // phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized, WordPress.Security.ValidatedSanitizedInput.MissingUnslash
 			$csstidy->set_cfg( 'optimise_shorthands', 0 );
+			$csstidy->set_cfg( 'preserve_leading_zeros', true );
 		}
 
 		$css = preg_replace( '/\\\\([0-9a-fA-F]{4})/', '\\\\\\\\$1', $css );
diff --git a/projects/plugins/jetpack/phpunit.xml.dist b/projects/plugins/jetpack/phpunit.xml.dist
index 10aa1e98f164..3cd084bd3b63 100644
--- a/projects/plugins/jetpack/phpunit.xml.dist
+++ b/projects/plugins/jetpack/phpunit.xml.dist
@@ -128,6 +128,9 @@
 		<testsuite name="seo-tools">
 			<directory prefix="test" suffix=".php">tests/php/modules/seo-tools</directory>
 		</testsuite>
+		<testsuite name="csstidy">
+			<directory prefix="test" suffix=".php">tests/php/modules/csstidy</directory>
+		</testsuite>
 	</testsuites>
 	<groups>
 		<exclude>
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
new file mode 100644
index 000000000000..a30b3bcc4d52
--- /dev/null
+++ b/projects/plugins/jetpack/tests/php/modules/csstidy/test-class.jetpack-csstidy.php
@@ -0,0 +1,64 @@
+<?php
+/**
+ * Class WP_Test_Jetpack_CSSTidy.
+ *
+ * @package automattic/jetpack
+ */
+
+require_jetpack_file( 'modules/custom-css/csstidy/class.csstidy.php' );
+
+/**
+ * Class WP_Test_Jetpack_CSSTidy
+ */
+class WP_Test_Jetpack_CSSTidy extends WP_UnitTestCase {
+
+	/**
+	 * The tested instance.
+	 *
+	 * @var csstidy
+	 */
+	public $instance;
+
+	/**
+	 * Sets up each test.
+	 *
+	 * @inheritDoc
+	 */
+	public function setUp() {
+		parent::setUp();
+		$this->instance = new csstidy();
+		$this->instance->set_cfg( 'optimise_shorthands', 0 );
+	}
+
+	/** Provides values for CSS preserve leading zeros patterns */
+	public function custom_preserve_leading_zeros_provider() {
+		// phpcs:ignore Squiz.PHP.CommentedOutCode.Found -- false positive
+		// 'test case description' => [ 'input', 'expected output', 'preserve_leading_zeros' ].
+		return array(
+			'test_removes_leading_zeros_by_default'       => array( 'marquee {line-height:0.7;opacity:0.05;background-color:rgba(255, 255, 255, 0.25);}', "marquee {\nline-height:.7;\nopacity:.05;\nbackground-color:rgba(255,255,255,0.25)\n}", false ),
+			'test_decimals_greater_than_one_unchanged_default' => array( 'blink {line-height:1.7;top:-100.55em;}', "blink {\nline-height:1.7;\ntop:-100.55em\n}", false ),
+			'test_removes_leading_zeros_by_default_units' => array( 'dfn {margin-left:-0.7px;top:0.55rem;line-height:0.3333;text-indent:-9999%}', "dfn {\nmargin-left:-.7px;\ntop:.55rem;\nline-height:.3333;\ntext-indent:-9999%\n}", false ),
+			'test_preserves_leading_zeros'                => array( 'aside {line-height:0.7;background-color:rgba(255, 255, 255, 0.25);opacity:0.05;}', "aside {\nline-height:0.7;\nbackground-color:rgba(255,255,255,0.25);\nopacity:0.05\n}", true ),
+			'test_preserves_leading_zeros_units'          => array( 'code {line-height:.70;margin-left:-00.70px;top:0.55rem;padding:0.3333%;}', "code {\nline-height:0.7;\nmargin-left:-0.7px;\ntop:0.55rem;\npadding:0.3333%\n}", true ),
+			'test_decimals_greater_than_one_unchanged_preserve_zeros' => array( 'blink {line-height:1.70;top:100.55em;margin-left:900px;}', "blink {\nline-height:1.7;\ntop:100.55em;\nmargin-left:900px\n}", false ),
+		);
+	}
+
+	/**
+	 * Test that leading zeros for decimals values are preserved/discarded as expected.
+	 *
+	 * @dataProvider custom_preserve_leading_zeros_provider
+	 *
+	 * @param string $input                  potential CSS values.
+	 * @param string $expected_output        what we expect csstidy to output.
+	 * @param bool   $preserve_leading_zeros the value of `preserve_leading_zeros` in csstidy's config.
+	 */
+	public function test_preserve_leading_zeros( $input, $expected_output, $preserve_leading_zeros ) {
+		$this->instance->set_cfg( 'preserve_leading_zeros', $preserve_leading_zeros );
+		$this->instance->parse( $input );
+		$this->assertEquals(
+			$expected_output,
+			$this->instance->print->plain()
+		);
+	}
+}