Let’s focus on this bit.
I don’t know we can do on the PlayCanvas side that is practical here. PlayCanvas renders in a Canvas that is a DOM object.
The developer can add any other DOM objects they like to the page. Help for HTML or CSS for these added DOMs is better served elsewhere generally because it is not specific to PlayCanvas.
I don’t see what other explanation there needs to be for this part or what you mean by PC-Atlas
I fully agree for the rendering pipeline and explanation. eg how the rendering layers work, shaders etc.