Puppeteer can be used to reach into the client context of an HTML Document during a test. This is super useful; We can use it to click on things and interact with the webpage like a client does, but in a predicable, repeatable way. Puppeteer can also take screenshots.
Code that can be run in more than one context, but usually referring to code executed by the Server and Browser. Fresh implements an Island Hydration strategy that renders html sever side first, and allows us to write client-side code only when absolutely necessary.