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Chapter 3. Units and Values > Summary - Pg. 78

Locator (URL). The difference is, for now, mostly semantic, but many authors are beginning to adopt the convention of referring to online addresses as URIs, not URLs. The specification still requires that URIs be declared with the form url(...) , though, so it's hard to know exactly what the point was of including a section in CSS2 about how CSS2 uses URIs instead of URLs. Summary Units and values cover a wide spectrum of areas, from length units 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. Interpreting relative URLs incorrectly, for example, has bedeviled many au- thors, and leads to an over-reliance 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 cir- cumstances will be discussed in the rest of the book, beginning with the CSS properties that describe ways to alter the way text is displayed.