By using selectors based on the document’s language, authors can create CSS rules that apply to a large number of similar elements just as easily as they can construct rules that apply in very narrow circumstances. The ability to group together both selectors and declarations keeps style sheets compact and flexible, which incidentally leads to smaller file sizes and faster download times. Thanks to inheritance and the cascade, authors are able to create sets of interlocking rules which, taken together, result in sophisticated document styles. This is all thanks to the fact that CSS is very deeply bound to the structure of documents, and that it uses this structure to determine how rules are applied, which rules should apply, and what elements should assume styles from their ancestors.
Selectors are the one thing that user agents almost have to get right, because the inability to correctly interpret selectors pretty much prevents a user agent from using CSS. However, the cascade and inheritance are different stories, and there have been some flaws in how they’re implemented. Navigator 4.x, for example, has a rough time with inheritance into tables, as well as other structures such as lists. You can clear up many of these problems by using very strictly correct HTML, but not all of them.
There are many CSS rules describing lengths, colors, or other things that can be expressed using specific units and values—and this is the subject (and title) of the next chapter.