Units and values cover a wide spectrum of areas, from length units to special keywords that describe relationships (such as smaller) to color units to the location of files (such as images). For the most part, units are the one area that user agents get almost totally correct; it's those few little bugs and quirks that get you, though. Navigator 4.x's failure to interpret relative URLs correctly, for example, has bedeviled many authors, and lead to an overreliance on absolute URLs. Colors are another area where user agents almost always do well, except for a few little quirks here and there. The vagaries of length units, however, far from being bugs, are an interesting problem for any author to tackle.
These units all have their advantages and drawbacks, depending on the circumstance in which they're used. We've already seen some of these (and the nuances of such circumstances will be discussed in the rest of the book), beginning with the CSS properties that describe ways to alter the way text is displayed.