Edit: Updated with the added context from your comment.
You could do this with a custom Page template. In your theme, create a new file. In this example, my theme will be “mytheme2015,” my new file is “template-holiday-card.php”:
in wp-content/themes/mytheme2015/template-holiday-card.php
<?php
/**
* Template Name: Holiday Card
*/
Now, once you have that template file created with a header like that, you can use it in the WordPress admin when you create a new Page type. The commented out “Template Name:” part tells WordPress what to call the template in the backend.
Note: if you don’t see the Page Attributes box, look in the Screen Options pulldown menu at the top of the page.
Now, you can put whatever code you want in template-holiday-card.php which (since it’s blank) won’t display anything by default.
If you want to allow your designer to input the HTML/CSS directly in WordPress through the admin, you could just have a call to the_content() in your template and allow them to edit the page’s content in HTML mode. Your resulting template would look something like:
<?php
/**
* Template Name: Holiday Card
*/
the_content();