screenshot-website | GitHub Action to capture screenshots | Continous Integration library

The important part is to take the screenshot

• exactly as the user sees the browser window. This rules out canvas-based questions and answers, which emulate the browser but cannot exactly replicate how it renders html and css
• without any non-standard plugins outside of plain, standards-compliant html5 and JS. This rules out Java applets, Flash, ActiveX, and kindly asking the user to press the "print-screen" button to then paste the image into a form.

I came across several questions on stackoverflow addressing this (see above), but did not find any answer that fulfilled both conditions. Instead of trying to add a partial answer for existing questions with slightly different requirements, I am asking this in a separate question instead.

I have so-far found a great explanation on how to acquire and stream pixels from a any window into a video element, using the Screen Capture API. However, I am missing the next step, where those pixels that are being displayed in a video element are converted into a jpg or png file that can be uploaded to a server.