Sadly a lot of browser features are inaccessible from non-https contexts.

jsheard 8 hours ago | parent | next [-]

Don't most (all?) browsers consider file:// and localhost to be secure for the sake of enabling those features?

Unfortunately, no. CORS will block this on Chrome and Firefox.

You need a local webserver. Or bundle everything in one html file.

or you could use something like caddy (https reverse-proxy, local cert), and have a dedicated ws running in the background that serves your files