Global Footer Content - What Content is valid/acceptable?

Just wondering what content is acceptable in the Global Footer Content Plugin? In a previous Forum post, Andrew made it sound like any code, would work. Specifically, he suggested.
add_action('wp_footer', 'additional_footer_content');
function additional_footer_content() {
echo '' . get_option('blogname') . '';
}

Since this is similar to what I am wanting to do, I tried this code. As expected it outputed this code to my page. I added <?php before and ?> after and got nothing.

My goal is to use add_action('wp_head', 'custom_css'); to add my css into the head as well as display my html in the footer. Is this possible? What am I missing.

Thanks.