I've been trying to implement some HTML code that accesses weather data in historical CSV files online, and perform maths on the data once I selectively extract it.
In the past, I've programmed in Python and had no problems doing this by using pycurl.Curl().  HTML is a complete nightmare in comparison:  XMLHttpRequest() does technically work, but web browsers automatically block access to all foreign URLs (because of the Same-Origin Policy).  Not good.
Any ideas and alternative approaches would be very helpful!
I'm pushing against the boundaries set forth in your question.. if you're simply wanting to do it in JavaScript, but outside the context of a browser, have you considered working in Node?
HTML - Access data from remote online CSV files