Because relative units are a painful and time consuming to deal with, thus counter-productive. How wide is the element that is 20em wide, that is contained by

Because relative units are a painful and time consuming to deal with, thus counter-productive. How wide is the element that is 20em wide, that is contained by an element that is 20em wide? Yes, that is a trick question (it depends on the browser, and the css of other elements on the page). How wide is an element that is 200px wide that is contained by an element that is 200px wide? The answer is straightforward - 200px. In terms of CSS these days, 1px != 1 screen pixel.