Ask HN: How to access user's computer from website using client-side program?

I want to create a website and a client-side program that would allow me to perform some arbitrary task (processing an image, computing a gradient, etc.) in the user's computer when they visit my website. It is important that I am using the user's CPU to perform the task rather than a separate server. After cursory research, I believe this is possible with a client-side program (such as a remote desktop connection), but I am not sure how to approach this problem the best way since I am new to networking/servers. How would you recommend I approach this problem and how would I build such a program? If you're interested in why I am trying to do this, look here: https://github.com/ktian08/decentralized-labor/blob/master/README.md