The body's border is 1em thick. The body doesn't have a font
size defined, so the em measurement here goes by the browser's
default font size, which may vary from browser to browser.
This font is 24 pixels tall and this div is 3 ems tall,
which results in a div 72 pixels tall (3 * 24 = 72). Its
border is 1em thick, or 24 pixels thick.