Source

File: wp-includes/class-wp-customize-widgets.php

public function customize_preview_init() {
		add_action( 'wp_enqueue_scripts', array( $this, 'customize_preview_enqueue' ) );
		add_action( 'wp_print_styles', array( $this, 'print_preview_css' ), 1 );
		add_action( 'wp_footer', array( $this, 'export_preview_data' ), 20 );
	}