https://www.google.com/search?q=javascript+math.random+vulnerability
Thank you for the comment. You probably right that the math.random is not secure. However actually my project doesn not need this function.
The idea is to create the private key manually, putting each bit to the cell of square 16x16 (256 bit in total). The most secure way is to flip a coin 256 times and fill every cell to receive the final 256 digit bin number. This 256bit number is immediately converted to the public key and bitcoin address with corresponding QR codes and WIF for private key.
math.random is used once to randomly fill all the 256 cells; it is also used in coin mode (0 or 1 for every cell). But of course I do not recommend to generate private keys with this random way (random function of the project). It was added just for educational purposes, or for more convinence making the starting point for key creation.
So the recommended ways to generate the key are:
1) Flip the coin 256 times and write down each outcome by filling the cell of the 16x16 square;
2) Use random, but with later manual intervention: received the filled square of the bits, manually change any cell you like to the opposite value, and do it manual changing as many times as you want.
3) Manually fill all the cells randomly chaning the cell and clicking the mouse to fill/unfill the cell.
4) Use visual patterns (visual in the context of bits presentation in the form of 16x16 square). This way could be done also for gift pusposes: for example draw a heart, generate the key with the address using this pattern, transfer some BTC to this address and present such key to the person you like.