Need a definition for what it means for something to be painted on screen #4825
Description
In order for the paint timing spec to define the first contentful paint, we need some CSS spec to define what it means for anything to be painted on screen.
Looks like we need to hook into the step 14 of updating the rendering step in the event loop processing model. There, we need some definition for what it means for a CSS box to be painted onto screen probably.