diff --git src/wp-includes/class-wp-customize-control.php src/wp-includes/class-wp-customize-control.php
index c45af67..7bed232 100644
--- src/wp-includes/class-wp-customize-control.php
+++ src/wp-includes/class-wp-customize-control.php
@@ -532,7 +532,7 @@ class WP_Customize_Control {
 					if ( ! empty( $this->description ) ) : ?>
 						<span class="description customize-control-description"><?php echo $this->description; ?></span>
 					<?php endif; ?>
-					<textarea rows="5" <?php $this->link(); ?>><?php echo esc_textarea( $this->value() ); ?></textarea>
+					<textarea rows="5" <?php $this->input_attrs(); ?> <?php $this->link(); ?>><?php echo esc_textarea( $this->value() ); ?></textarea>
 				</label>
 				<?php
 				break;
diff --git src/wp-includes/class-wp-customize-manager.php src/wp-includes/class-wp-customize-manager.php
index 229be36..bc9e111 100644
--- src/wp-includes/class-wp-customize-manager.php
+++ src/wp-includes/class-wp-customize-manager.php
@@ -3393,6 +3393,9 @@ final class WP_Customize_Manager {
 			'type'     => 'textarea',
 			'section'  => 'custom_css',
 			'settings' => array( 'default' => $custom_css_setting->id ),
+			'input_attrs' => array(
+				'placeholder' => __( "/*\nYou can add your own CSS here.\n\nClick the help icon above to learn more.\n*/" ),
+			)
 		) );
 	}