I believe all browsers block execution of the script (and rendering of other proceeding content) so his code should work generally.
Even the latest browsers that do not block further object (scripts requests during the download and execution of the script will execute scripts sequentially, so his check for "is jQuery present" will not fire until the external script has either returned and executed (so the check passes, and nothing else happens) or errored (so jQuery is not present and the document.write executes, making it load from the local resource).
Even the latest browsers that do not block further object (scripts requests during the download and execution of the script will execute scripts sequentially, so his check for "is jQuery present" will not fire until the external script has either returned and executed (so the check passes, and nothing else happens) or errored (so jQuery is not present and the document.write executes, making it load from the local resource).