Cascading Style Sheets: The Definitive Guide

Property Conventions

Throughout this book, there are boxes that break down a given CSS property. These have been reproduced practically verbatim from the CSS specifications, but some explanation of the syntax is in order.

Throughout, the allowed values for each property are listed with a syntax like the following:

Value: [ <length> | thick | thin ]{1,4}

Value: [<family-name> , ]* <family-name>

Value: <url>? <color> [ / <color> ]?

Value: <url> || <color>

Any words between < and > give a type of value, or a reference to another property. For example, the property font will accept values which actually belong to the property font-family. This is denoted by using the text “<font-family>.” Any words presented in constant width text are keywords that must appear literally, without quotes. The forward slash ( / ) and the comma ( , ) must also be used literally.

Several keywords strung together means that all of them must occur—in the given order. For example, help me would mean that the property must use those keywords in that exact order.

If a vertical bar ( X | Y ) separates alternatives, then any one of them must occur. A vertical double bar ( X || Y ) means that either X or Y, or both, must occur, but they may appear in any order. Brackets ( [...] ) are for grouping things together. Juxtaposition is stronger than the double bar, and the double bar is stronger than the bar. Thus “V W | X || Y Z” is equivalent to " [ V W ] | [ X || [ Y Z ]] .”

Every word or bracketed group may be followed by one of the following modifiers:

• An asterisk (*) indicates that the preceding value or bracketed group is repeated zero or more times. Thus, bucket* means that the word bucket can be used any number of times, including zero. There is no defined upper limit on the number of times it can be used.

• A plus (+) indicates that the preceding value or bracketed group is repeated one or more times. Thus, mop+ means that the word mop must be used at least once, and potentially many more times.

• A question mark (?) indicates that the preceding value or bracketed group is optional. For example, [ pine tree ]? means that the words pine tree need not be used (although they must appear in that exact order if they are used).

• A pair of numbers in curly braces ({M,N}) indicates that the preceding value or bracketed group is repeated at least M and at most N times. For example, ha{1,3} means that there can be one, two, or three instances of the word ha.

Following are some examples:

give || me || liberty

At least one of the three words must be used, and all of them can be used in any order. For example, give liberty, give me, liberty me give, and give me liberty are all valid interpretations of this example.

[I | am]? the || walrus

Either, but not both, of the words I or am may be used, but use of either is optional. In addition, either the or walrus, or both, must follow. Thus, you could construct I the walrus, am walrus the, am the, I walrus, walrus the, and so forth.

koo + ka-choo

One or more instances of koo must be followed by ka-choo. Therefore, koo koo ka-choo, koo koo koo ka-choo, and koo ka-choo are all legal. The number of koos is potentially infinite, although there are bound to be implementation-specific limits.

I really{1,4}* [love | hate] [Microsoft | Netscape]

The all-purpose web designer’s opinion-expresser. This can be interpreted as I love Netscape, I really love Microsoft, and similar expressions. Anywhere from zero to four really s may be used. You also get to pick between love and hate, even though only love was shown in this example.

[[[Alpha || Baker || Cray],]{2,3} and ] Delphi

This is a potentially long and complicated expression. One possible result would be Alpha, Cray, and Delphi. Another is Alpha Baker, Cray Alpha, Baker Cray Alpha, and Delphi. The comma is placed due to its position within the nested bracket groups.