In terms of the actual HTML structure of your website, anything placed into the Custom HTML area will be placed directly after your website's opening <body> tag:
<html> <head> Site title, meta tags, and favicon are all determined in your website's admin panel: admin panel → Site Details admin panel → Settings → favicon </head> <body> <customhtml> Anything inside your Custom HTML area is placed here. </customhtml> All Cargo-generated HTML (text nav, content, thumbnails, etc) is placed directly after the Custom HTML. </body> </html>
Since head and body tags are automatically placed into each website, there is no need to place them inside the Custom HTML area.
The actual source code of your Cargo design can be modified through the use of the Cargo Developer Tools. The source code of non-Developer Designs can't be modified.