Source

File: wp-includes/class.wp-styles.php

public function __construct() {
		if (
			function_exists( 'is_admin' ) && ! is_admin()
		&&
			function_exists( 'current_theme_supports' ) && ! current_theme_supports( 'html5', 'style' )
		) {
			$this->type_attr = " type='text/css'";
		}

		/**
		 * Fires when the WP_Styles instance is initialized.
		 *
		 * @since 2.6.0
		 *
		 * @param WP_Styles $this WP_Styles instance (passed by reference).
		 */
		do_action_ref_array( 'wp_default_styles', array( &$this ) );
	}