JavaScript: Secure Random Number Generation

I need to generate secure random numbers using JS.
This random number generation must be as secure as possible(as random as possible).
So I clearly can't use Math.random() here.
AFAIK my best choice is to use Crypto API's crypto.getRandomValues() method.

I read that Crypto API is 'as secure as it gets'(apart from a 'true' RNG) and seeded with values taken from a platform-specific RNG like /dev/urandom(in unix).

1. Is crypto.getRandomValues() really secure? Are there any flows I should worry about?

2. Secure or insecure, should I take Crypto.getRandomValues()'s randomness any further? Like,

I can use crypto.getRandomValues() again to get a random value from the first result of crypto.getRandomValues().

OR

I can implement my own RNG that uses user mouse movements/time/window width/height as seed.
Then use random numbers from that RNG to randomly select a number from the result of crypto.getRandomValues().

Any insight would be appreciated.