Your experience will not be rendered, but normal default content should still be processed and rendered as expected.
at.js is a library to support your experience, it is absolutely required to be in the header.
The modifications to page content YOU control via the target interface.
as far as i know it is a "blocking script", meaning it has to fully execute before the next script can begin execution, but Adobe, please correct me if i'm wrong. Even then, its a fully tested and QA cycled script, I personally haven't seen it fail or have issues with the library yet on our platform. We are using at.js @ v 1.2.0