I don't understand why SCRIPT elements doesn't have a "depends" attribute that list id names to other SCRIPT that must have finished before this script executes.

Isn't that a huge issue if you intend to defer/async a script element for a js-library that other scripts depend on being there?

The Mozilla bug report doesn't make clear what the actual limitations are of this implementation - like how you can affect the DOM while running asynchronously. Any ideas about what actually happens?