Might be worth looking at the Bitcoin Core's RNG code for me but the RNG of JS which would be `crypto.getRandomBytes()` is already used for wallets like Metamask which more than a million people use to store crypto.
Metamask is running in the browser. No secure software does that, if you were to compare it with something like electrum, because it introduces more risks than necessary. Also, suggesting it because many use it holds as much water as suggesting custodial exchanges because millions use it, ignoring all the incidents of compromises there have been.