I am building a React.JS application that is supposed to encrypt and decrypt data.

I'm using the subtlecrypto api to create RSA key pairs. I want to store them, preferably in the browser, in a way the user can re-use them when he returns to the site.

I absolutely do NOT want to store them in any kind of server or online storage, because that would defeat the purpose of the whole exercise.

Is there a dedicated facility to store secrets like that? If not, what is the most secure option available?