On my site, when I implement at.js in head, it didn't work. But if I implement mbox.js in head, it worked.
I'm just wondering why it will happen, for at.js is a new method?
The at.js fires the global mbox request from the HEAD of the document and so HTML content can't be rendered there. Depending on use case there should be several options available for adding HTML with the visual composer:
The global mbox was designed for the visual composer so you may find those options work better for you. However, if you find that you must use the form composer you have these options available.
Here's a help doc on adding a regional mbox to your page: mboxCreate(mbox,params)