CSS Length Reference To bound, or not to bound, that is the question It's quite a reasonable request, really, and it's already been implemented for HTML. That is, length bounding. It makes little sense to let users define text blocks that have a font-size of 63,360 inches (that's a mile, by the way) or a width of forty-fold the parent container. But it's a little more complicated then that. There are multiple units one can use, and we have to a little unit conversion to get things working. Here's what we have: Absolute: 1 in ~= 2.54 cm 1 cm = 10 mm 1 pt = 1/72 in 1 pc = 12 pt Relative: 1 em ~= 10.0667 px 1 ex ~= 0.5 em, though Mozilla Firefox says 1 ex = 6px 1 px ~= 1 pt Watch out: font-sizes can also be nested to get successively larger (although I do not relish having to keep track of context font-sizes, this may be necessary, especially for some of the more advanced features for preventing things like white on white).