Quick answer: Godot 4 HTML5 game with multithreading enabled refusing to start in browser? Pthreads require COOP/COEP headers - configure web server or use single-thread export.
Game runs in editor; itch.io upload shows blank page. Console: SharedArrayBuffer is undefined.
Set COOP/COEP headers
Web server config: Cross-Origin-Opener-Policy: same-origin, Cross-Origin-Embedder-Policy: require-corp. SharedArrayBuffer becomes available.
Or export without threads
Export preset > HTML5 > Thread Support = off. Larger binary; no header requirement.
Use itch's SharedArrayBuffer setting
itch.io project page has a checkbox for SharedArrayBuffer support. Toggle it for threaded exports.
“Threaded HTML5 is browser-modern. Modern requires deployment cooperation.”
For itch.io distribution, single-thread is the safe default. Reserve threaded for self-hosted where you control headers.