Allows developer/client only customization previews with WordPress.

gistfile1.php

<?php
/*
 *
 * @author: Byron Rode
 * @description: Allows developer/client only customization previews with WordPress. Uses a simple
 * cookie to handle previews, so don't go showing sensitive data. PS. I am NOT responsible
 * if you do.
 * @license: Free as in Beer
 *
 */
 
// This handles the cookie creation/deletion based on a query 
// string variable (preview=true|false).

if( !function_exists( 'tgm_dev_preview' ) ) {
  function tgm_dev_preview()
	{
		
		if( isset($_GET['preview']) && $_GET['preview'] == 'true' ) {
			if( !isset($_COOKIE['tgm_show_preview']) || isset($_COOKIE['tgm_show_preview']) && $_COOKIE['tgm_show_preview'] == 'false' ) {
				setcookie('tgm_show_preview', 'true', time() + 3600, '/');
			}else{
				setcookie('tgm_show_preview', '', time() - 3600, '/');
			}

			wp_safe_redirect( '/' );
			exit;
		}
		
		if( isset($_GET['preview']) && $_GET['preview'] == 'false' ) {
			setcookie('tgm_show_preview', '', time() - 3600, '/');
			wp_safe_redirect( '/' );
			exit;
		}
		
		return;
		
	}
}
add_action('template_redirect', 'tgm_dev_preview');

// This adds CSS specific to your priview. You could do the same for JS if you needed, 
// by replacing css with js, and by changing wp_enqueue_style to wp_enqueue_script. Remember to add
// dependencies for JS.

if( !function_exists( 'tgm_dev_preview_css' ) ) {
	function tgm_dev_preview_css()
	{		
		if( !$_COOKIE['tgm_show_preview'] || isset($_COOKIE['tgm_show_preview']) && $_COOKIE['tgm_show_preview'] == 'false' ) {
			wp_enqueue_style('tgm-dev-css', get_stylesheet_directory_uri() . '/dev.css');
		}
	}
}
add_action('wp_print_styles', 'tgm_dev_preview_css', 100);

// This adds the ability to show/hide specific content from within the WP editor.
// All content between [tgm_dev_preview] content here [/tgm_dev_preview] will 
// be hidden, by default. The trigger will enable it.

if( !function_exists( 'tgm_dev_preview_shortcode' ) ) {
	function tgm_dev_preview_shortcode( $atts, $content = null )
	{
		if( isset($_COOKIE['tgm_show_preview']) && $_COOKIE['tgm_show_preview'] == 'true' ) {
			$content = '';
		}
		return str_replace('<br />', '', $content);
	}
}
add_shortcode('tgm_dev_preview', 'tgm_dev_preview_shortcode');