The problem is that a cryptographicly secure rng generates all numbers with equal probability. Non-cryptographic rng s have no such guarantee, so even given a t

The problem is that a cryptographicly secure rng generates all numbers with equal probability. Non-cryptographic rng s have no such guarantee, so even given a truly random seed and a single value taken from the rng, one might still know something about the distribution of the rng's output that will allow them to know what values are more likely to have been produced, therefore allowing them to launch a brute force attack using the most likely values first, reducing the number of attempts they can expect to need in order to find the correct key. That is not to say that one cannot use javascript for secure key generation, however one must make sure that the algorithm used to convert the truly random entropy into a useful random number is secure.