Is possible to implement a Web Cryptography API custom provider?

I’m reading some basic info about Web Cryptography API and I’m wondering if is possible to implement some crypto provider (C/C++ library or something) with some extra algorithms or is mandatory to use the ones “embedded” with the web browser. I have finded articles about the security and tutorials about how to use it but nothing about custom implementation. I don’t know if it uses Operating System libraries or only web browser libraries, if should be used “as is”… Some reference or clarification is appreciated.