Eric A. Meyer - CSS: The Definitive Guide

Throughout this book, there are boxes that break down a given CSS propertys details, including what values are permitted. 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: #

Value:

Value: ? [ / ]?

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

Any italicized words between < and > give a type of value, or a reference to another propertys values. For example, the property font accepts values that originally belong to the property font-family. This is denoted by using the text . Similarly, if a value type like a color is permitted, it will be represented using .

Any words presented in constant width are keywords that must appear literally, without quotes. The forward slash (/) and the comma (,) must also be used literally.

There are a number of ways to combine components of a value definition:

• Two or more keywords strung together with only space separating them 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 order.

• If a vertical bar separates alternatives (X | Y), then any one of them must occur, but only one. Given [ X | Y | Z ], then any one of X, Y, or Z is permitted.

• A vertical double bar (X Y) means that X, Y, or both must occur, but they may appear in any order. Thus: X, Y, X Y, and Y X are all valid interpretations.

• A double ampersand (X && Y) means both X and Y must occur, though they may appear in any order. Thus: X Y or Y X are both valid interpretations.

• Brackets ([]) are for grouping things together. Thus [please help me] do this means that the words please, help, and me can appear in any order, though each appear only once. do this must always appear, with those words in that order. Some examples: please help me do this, help me please do this, me please help do this.

Every component or bracketed group may (or may not) be followed by one of these 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 upper limit defined 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.

• An octothorpe (#) indicates that the preceding value or bracketed group is repeated one or more times, separated by commas as needed. Thus, floor# can be floor, floor, floor, floor, and so on. This is most often used in conjunction with bracketed groups or value types.

• 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 order if they are used).

• An exclamation point (!) indicates that the preceding value or bracketed group is required, and thus must result in at least one value, even if the syntax would seem to indicate otherwise. For example, [