Async just means that there is a script, which, some time after the page is technically "fully loaded", starts to load some more! That's useful for updates or actions that should occur without a complete page reload (which would discard the "user state").
It doesn't necessarily mean you see an URL list. It can be implemented by storing the list in Javascript code - and you don't "see" that Javascript code.
The method you mention (mis)uses HTML Tags as datastore for the URLs list.
It doesn't necessarily mean you see an URL list. It can be implemented by storing the list in Javascript code - and you don't "see" that Javascript code.
The method you mention (mis)uses HTML Tags as datastore for the URLs list.