In Chrome, window.crypto is read-only:

> window.crypto

< Crypto {subtle: SubtleCrypto}

> window.crypto = "hi!"

< "hi!"

> window.crypto

< Crypto {subtle: SubtleCrypto}

But not anything underneath, including getRandomValues(), as you write. A recent issue about this [1] on the WebCrypto spec itself was closed with 'wontfix' because in their view, polyfilling web APIs is a common and accepted practice.

[1] https://github.com/w3c/webcrypto/issues/107

Shouldn't all pollyfills check only override the functions if they're missing?

What if it's present but the implementation is incomplete?

E.g. an extra optional parameter.