Units and values cover a wide spectrum of areas, from length units to special keywords that describe effects (such as underline) 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, but it's those few little bugs and quirks that can get you. Navigator 4.x's failure to interpret relative URLs correctly, for example, has bedeviled many authors and led 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 upon the circumstances in which they're used. We've already seen some of these circumstances, and their nuances will be discussed in the rest of the book, beginning with the CSS properties that describe ways to alter how text is displayed.