How websites or tools like jsfiddle are implemented or work?

How websites or tools like jsfiddle are implemented or work?

I was not sure to ask the question here. I looked at the help center again
and found that yes, We can ask about a software algorithm or software
tools commonly used by programmers.
Anyway my question is about websites or tools that help troubleshooting
some combination of HTML, CSS, and JavaScript.
How websites or tools like jsfiddle are implemented or work?
What are steps to combine HTML, CSS, and JavaScript and also a library
like e.g. jquery?
How can evaluate javascript in a good and optimized way?