In a web browser, typically the layout engine generates a "display list" which is a list of drawing commands that includes all of the rectangles, borders, text,

In a web browser, typically the layout engine generates a "display list" which is a list of drawing commands that includes all of the rectangles, borders, text, etc. It hands the display list to a graphics engine to be rasterized. The graphics code may optimize the display list before rasterizing it, doing things like removing items that are completely occluded.