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[quote]The correct answer, though, is 2.
This is a lesson that was drilled into me a long time ago, when I was first learning CSS. Always specify line-height as a unitless number; that way, descendent elements that specify a different font-size will inherit that number rather than a fixed line height.
Let’s say the page has a default font-size of 12pt, but it also contains a heading with a font-size of 24pt. If you set the line-height of the body to 2em (or 200%), then you’ll get a line height of exactly 24pt (twice the body’s font-size) everwhere in the document—even in that heading. The heading will therefore be single-spaced, not double-spaced!
Setting line-height to 2 instead tells the browser to preserve the font-size/line-height ratio even when the font size changes. The line height for the body will be 24pt, but for the heading’s 24pt font, the line height will automaticlly increase to 48pt.[/quote]