Newer Godot releases allow for HTML5 exports using multithreading. These exports however make use of the SharedArrayBuffer Javascript feature. Unfortunately, in Firefox (and from may onwards in Chrome) this functionality is only available to pages that are cross-origin isolated. This can only be done by itch.io itself. I suggest itch.io implements the thse headers to support this feature.
Cross-Origin-Opener-Policy: same-origin Cross-Origin-Embedder-Policy: require-corp
Links with more info:
https://developer.chrome.com/blog/enabling-shared-array-buffer/
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects...