CSS: The Definitive Guide, 5th Edition

Chapter 5. Values and Units

In this chapter, we’ll tackle features that are the basis for almost everything you can do with CSS: the units that affect the colors, distances, and sizes of a whole host of properties, as well as the units that help define those values. Without units, you couldn’t declare that an image should have 10 pixels of blank space around it, or that a heading’s text should be a certain size. By understanding the concepts put forth here, you’ll be able to learn and use the rest of CSS much more quickly.

Keywords, Strings, and Other Text Values

Everything in a stylesheet is text, but certain value types directly represent strings of text as opposed to, say, numbers or colors. Included in this category are URLs and, interestingly enough, images.

Keywords

For those times when a value needs to be described with a word of some kind, CSS has keywords. A common example is the keyword none, which is distinct from 0 (zero). Thus, to remove the underline from links in an HTML document, you would write the following:

a[href] {text-decoration: none;}

Similarly, if you want to force underlines on the links, you would use the keyword underline instead of none.

If a property accepts keywords, its keywords will be defined only for the scope of that property. If two properties use the same word as a keyword, the behavior of the keyword for one property will not necessarily be shared with the other. As an example, normal, as defined for letter-spacing, means something very ...