I have no idea how to debug this, thoughts?
I just copied and pasted, there are 4 files, 1 simple CSS styling of the left upper corner card with text and a button, 1 HTML file with the content and some divs, 1 javascript handler for the CSS and one for the HTML (and I have no idea what they do). I do know CSS and HTML well but I don’t know much about javascript.
You have an Entity called html+css, with a script component that has 2 scripts - The htmlHandler and the cssHandler. These scripts expects a script attribute - the first expects an HTML asset and the second a CSS asset. So in the Editor you need to drag and drop those assets to the respective slots of the script component.
I also recommend going through the user manual to get accustomed to PlayCanvas: