What HTML tags and CSS elements can be used in the custom templates?

The head and body tags in the email template are present, they are just hidden. You can think of all the HTML entered in the editor as going between the <body> </body> tags.

In the case of the packing slip templates, the head and body tags, as well as some table and styling information, is also present but hidden. 

However, we've made the complete default packing slip template available to download to assist in any customization you might want to make. You can download the sample html file from our Packing Slips help article. Once you open the file in a browser you can then view the source code and see the entire HTML and CSS for the default packing slip template, including any hidden elements.