diff --git a/src/wp-includes/class-wp-styles.php b/src/wp-includes/class-wp-styles.php
index 9b210b2df9d30..6d47bc15d56fd 100644
--- a/src/wp-includes/class-wp-styles.php
+++ b/src/wp-includes/class-wp-styles.php
@@ -183,12 +183,11 @@ public function do_item( $handle, $group = false ) {
 		$inline_style = $this->print_inline_style( $handle, false );
 
 		if ( $inline_style ) {
-			$inline_style_tag = sprintf(
-				"<style id='%s-inline-css'%s>\n%s\n</style>\n",
-				esc_attr( $handle ),
-				$this->type_attr,
-				$inline_style
-			);
+			$processor = new WP_HTML_Tag_Processor( "<style{$this->type_attr}></style>\n" );
+			$processor->next_tag();
+			$processor->set_attribute( 'id', "{$handle}-inline-css" );
+			$processor->set_modifiable_text( "\n{$inline_style}\n" );
+			$inline_style_tag = $processor->get_updated_html();
 		} else {
 			$inline_style_tag = '';
 		}
@@ -364,12 +363,11 @@ public function print_inline_style( $handle, $display = true ) {
 			return $output;
 		}
 
-		printf(
-			"<style id='%s-inline-css'%s>\n%s\n</style>\n",
-			esc_attr( $handle ),
-			$this->type_attr,
-			$output
-		);
+		$processor = new WP_HTML_Tag_Processor( "<style{$this->type_attr}></style>\n" );
+		$processor->next_tag();
+		$processor->set_attribute( 'id', "{$handle}-inline-css" );
+		$processor->set_modifiable_text( "\n{$output}\n" );
+		echo $processor->get_updated_html();
 
 		return true;
 	}
diff --git a/src/wp-includes/customize/class-wp-customize-custom-css-setting.php b/src/wp-includes/customize/class-wp-customize-custom-css-setting.php
index aab0e475304ea..ebe61fbf270f6 100644
--- a/src/wp-includes/customize/class-wp-customize-custom-css-setting.php
+++ b/src/wp-includes/customize/class-wp-customize-custom-css-setting.php
@@ -145,14 +145,14 @@ public function value() {
 	}
 
 	/**
-	 * Validate a received value for being valid CSS.
+	 * Validate a received value for being safe HTML STYLE tag contents.
 	 *
-	 * Checks for imbalanced braces, brackets, and comments.
 	 * Notifications are rendered when the customizer state is saved.
 	 *
 	 * @since 4.7.0
 	 * @since 4.9.0 Checking for balanced characters has been moved client-side via linting in code editor.
 	 * @since 5.9.0 Renamed `$css` to `$value` for PHP 8 named parameter support.
+	 * @since 7.0.0 Relaxed to only check for safe HTML STYLE tag contents.
 	 *
 	 * @param string $value CSS to validate.
 	 * @return true|WP_Error True if the input was validated, otherwise WP_Error.
@@ -163,8 +163,16 @@ public function validate( $value ) {
 
 		$validity = new WP_Error();
 
-		if ( preg_match( '#</?\w+#', $css ) ) {
-			$validity->add( 'illegal_markup', __( 'Markup is not allowed in CSS.' ) );
+		/**
+		 * Check for a closing STYLE tag inside the CSS.
+		 *
+		 * STYLE tags are processed using the "generic raw text parsing algorithm." They contain
+		 * raw text up until a matching closing tag.
+		 *
+		 * @see https://html.spec.whatwg.org/multipage/parsing.html#generic-raw-text-element-parsing-algorithm
+		 */
+		if ( false !== stripos( $css, '</style' ) ) {
+			$validity->add( 'illegal_markup', __( 'CSS must not contain possible closing STYLE tag "</style".' ) );
 		}
 
 		if ( ! $validity->has_errors() ) {
diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php
index 908ebe4bcc777..6a3405166a3d5 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php
@@ -659,16 +659,25 @@ public function get_theme_items( $request ) {
 	/**
 	 * Validate style.css as valid CSS.
 	 *
-	 * Currently just checks for invalid markup.
+	 * Currently just checks that CSS will not break an HTML STYLE tag.
 	 *
 	 * @since 6.2.0
 	 * @since 6.4.0 Changed method visibility to protected.
+	 * @since 7.0.0 Relaxed to only check for safe HTML STYLE tag contents.
 	 *
 	 * @param string $css CSS to validate.
 	 * @return true|WP_Error True if the input was validated, otherwise WP_Error.
 	 */
 	protected function validate_custom_css( $css ) {
-		if ( preg_match( '#</?\w+#', $css ) ) {
+		/**
+		 * Check for a closing STYLE tag inside the CSS.
+		 *
+		 * STYLE tags are processed using the "generic raw text parsing algorithm." They contain
+		 * raw text up until a matching closing tag.
+		 *
+		 * @see https://html.spec.whatwg.org/multipage/parsing.html#generic-raw-text-element-parsing-algorithm
+		 */
+		if ( false !== stripos( $css, '</style' ) ) {
 			return new WP_Error(
 				'rest_custom_css_illegal_markup',
 				__( 'Markup is not allowed in CSS.' ),
diff --git a/src/wp-includes/script-loader.php b/src/wp-includes/script-loader.php
index 56986c3d80a79..e968ecb693758 100644
--- a/src/wp-includes/script-loader.php
+++ b/src/wp-includes/script-loader.php
@@ -2417,10 +2417,12 @@ function _print_styles() {
 		echo "<link rel='stylesheet' href='" . esc_attr( $href ) . "'{$type_attr} media='all' />\n";
 
 		if ( ! empty( $wp_styles->print_code ) ) {
-			echo "<style{$type_attr}>\n";
-			echo $wp_styles->print_code;
-			echo sprintf( "\n/*# sourceURL=%s */", rawurlencode( $concat_source_url ) );
-			echo "\n</style>\n";
+			$processor = new WP_HTML_Tag_Processor( "<style{$type_attr}></style>" );
+			$processor->next_tag();
+			$style_tag_contents = "\n{$wp_styles->print_code}\n"
+				. sprintf( "/*# sourceURL=%s */\n", rawurlencode( $concat_source_url ) );
+			$processor->set_modifiable_text( $style_tag_contents );
+			echo $processor->get_updated_html();
 		}
 	}
 
@@ -3217,7 +3219,10 @@ function wp_enqueue_block_support_styles( $style, $priority = 10 ) {
 	add_action(
 		$action_hook_name,
 		static function () use ( $style ) {
-			echo "<style>$style</style>\n";
+			$processor = new WP_HTML_Tag_Processor( "<style></style>\n" );
+			$processor->next_tag();
+			$processor->set_modifiable_text( $style );
+			echo $processor->get_updated_html();
 		},
 		$priority
 	);
diff --git a/src/wp-includes/theme.php b/src/wp-includes/theme.php
index 558bea6ae9e02..b8d35a255d35c 100644
--- a/src/wp-includes/theme.php
+++ b/src/wp-includes/theme.php
@@ -1953,11 +1953,13 @@ function _custom_background_cb() {
 
 		$style .= $image . $position . $size . $repeat . $attachment;
 	}
-	?>
-<style<?php echo $type_attr; ?> id="custom-background-css">
-body.custom-background { <?php echo trim( $style ); ?> }
-</style>
-	<?php
+
+	$processor = new WP_HTML_Tag_Processor( "<style{$type_attr} id=\"custom-background-css\"></style>" );
+	$processor->next_tag();
+
+	$style_tag_content = 'body.custom-background { ' . trim( $style ) . ' }';
+	$processor->set_modifiable_text( "\n{$style_tag_content}\n" );
+	echo $processor->get_updated_html();
 }
 
 /**
@@ -1967,17 +1969,18 @@ function _custom_background_cb() {
  */
 function wp_custom_css_cb() {
 	$styles = wp_get_custom_css();
-	if ( $styles || is_customize_preview() ) :
-		$type_attr = current_theme_supports( 'html5', 'style' ) ? '' : ' type="text/css"';
-		?>
-		<style<?php echo $type_attr; ?> id="wp-custom-css">
-			<?php
-			// Note that esc_html() cannot be used because `div &gt; span` is not interpreted properly.
-			echo strip_tags( $styles );
-			?>
-		</style>
-		<?php
-	endif;
+	if ( ! $styles && ! is_customize_preview() ) {
+		return;
+	}
+
+	$processor = new WP_HTML_Tag_Processor( '<style></style>' );
+	$processor->next_tag();
+	if ( ! current_theme_supports( 'html5', 'style' ) ) {
+		$processor->set_attribute( 'type', 'text/css' );
+	}
+	$processor->set_attribute( 'id', 'wp-custom-css' );
+	$processor->set_modifiable_text( "\n{$styles}\n" );
+	echo $processor->get_updated_html();
 }
 
 /**
diff --git a/tests/phpunit/tests/rest-api/rest-global-styles-controller.php b/tests/phpunit/tests/rest-api/rest-global-styles-controller.php
index 59986e597c71b..ab4395db02c2f 100644
--- a/tests/phpunit/tests/rest-api/rest-global-styles-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-global-styles-controller.php
@@ -650,6 +650,7 @@ public function test_update_item_valid_styles_css() {
 	/**
 	 * @covers WP_REST_Global_Styles_Controller::update_item
 	 * @ticket 57536
+	 * @ticket 64418
 	 */
 	public function test_update_item_invalid_styles_css() {
 		wp_set_current_user( self::$admin_id );
@@ -659,7 +660,9 @@ public function test_update_item_invalid_styles_css() {
 		$request = new WP_REST_Request( 'PUT', '/wp/v2/global-styles/' . self::$global_styles_id );
 		$request->set_body_params(
 			array(
-				'styles' => array( 'css' => '<p>test</p> body { color: red; }' ),
+				'styles' => array(
+					'css' => '</style would close the containing element and is not allowed.',
+				),
 			)
 		);
 		$response = rest_get_server()->dispatch( $request );
@@ -826,4 +829,32 @@ public function test_global_styles_route_args_schema() {
 		$this->assertArrayHasKey( 'type', $route_data[0]['args']['id'] );
 		$this->assertSame( 'integer', $route_data[0]['args']['id']['type'] );
 	}
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::update_item
+	 * @ticket 64418
+	 */
+	public function test_update_allows_valid_css_with_more_syntax() {
+		wp_set_current_user( self::$admin_id );
+		if ( is_multisite() ) {
+			grant_super_admin( self::$admin_id );
+		}
+		$request = new WP_REST_Request( 'PUT', '/wp/v2/global-styles/' . self::$global_styles_id );
+		$css     = <<<'CSS'
+@property --animate {
+	syntax: "<custom-ident>";
+	inherits: true;
+	initial-value: false;
+}
+CSS;
+		$request->set_body_params(
+			array(
+				'styles' => array( 'css' => $css ),
+			)
+		);
+
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+		$this->assertSame( $css, $data['styles']['css'] );
+	}
 }