The importance of this cannot be understated - your users will thank you for being considerate of their preferences. BIG's text is 1.2 times as big as P's. P, lacking a font size declaration, inherits BODY's value. BODY also lacks a font size declaration, so inherits from HTML, which in turn lacks a font size declaration. The pixel is a much underrated unit, suffering greatly from ignorance as to its meaning.
Many people assume that the pixel is as bad as real-world lengths.
As a result, although other units are always better in theory, in practice they are too badly broken in one browser or another, and therefore the pixel is the only unit that is of any use in style sheets intended for general consumption . 17px on their higher resolution as someone else is on their low-res machine. Best yet, the CSS specification states that if the output device pixel differs greatly from the specification's reference pixel, it should be scaled. This scaling is carried out by print drivers before printing. However, no browser performs any kind of scaling when displaying on screen, principally because they have no information about the true ppi value - to scale using the OS' ppi value would be false, since it never bears any relation to the true value.