Meanderings on a new Settings API

gistfile1.php

<?php

/**
 * A singular settings object. By itself, it only describes the relationship of 
 * the setting to any WordPress objects (post type, taxonomy term, the WP instance)
 * that it relates to. 
 */
$setting = new WP_Setting(
	array(
		'key' => 'title_color',
		'for' => array(
			'post_type' => array( 'post', 'page' ),
			'taxonomy'  => array( 'category' ),
			'site',
			'network'
		),
	)
);

/**
 * A settings group is a bucket for multiple settings to be put in.
 * 
 * This is strictly a data relationship. Other APIs/code can tap in to the group,
 * but the group by itself does nothing.
 */
$group = new WP_Settings_Group();
$group->add_setting( $setting );

/**
 * An instance of a meta box, which will output on the screen(s) given.
 */
$metabox = new WP_Meta_Box( array(
	'screen' => 'post'
) );

/**
 * Relate a settings group to a metabox. 
 * 
 * The metabox should respect the type of each setting for form field
 * outputs, and pass along data to methods in the class for saving, 
 * validating, and sanitizing data.
 */
$metabox->add_settings_group( $group );

$setting = wp_register_setting( array( 
	'key'  => 'custom_css',
	'type' => 'text',
	'for'  => 'site',
	'sanitization_callback' => 'thing',
	'validation_callback' => 'thing'
) );