W3C CSS2 Cascading Style Sheets, level 2 - instructional media + ...

A user agent conforming to a future CSS specification may accept one or
more of the other rules as well.
Invalid at-keywords. User agents must ignore [p. 42] an invalid at-keyword
together with everything following it, up to and including the next semicolon
(;) or block ({...}), whichever comes first. For example, consider the following:
@three-dee {
@background-lighting {
azimuth: 30deg;
elevation: 190deg;
}
H1 { color: red }
}
H1 { color: blue }
The ’@three-dee’ at-rule is not part of CSS2. Therefore, the whole at-rule
(up to, and including, the third right curly brace) is ignored. [p. 42] A CSS2
user agent ignores [p. 42] it, effectively reducing the style sheet to:
H1 { color: blue }
4.3 Values
4.3.1 Integers and real numbers
Some value types may have integer values (denoted by ) or real
number values (denoted by ). Real numbers and integers are specified
in decimal notation only. An consists of one or more digits "0" to "9". A
can either be an , or it can be zero or more digits followed by
a dot (.) followed by one or more digits. Both integers and real numbers may be
preceded by a "-" or "+" to indicate the sign.
Note that many properties that allow an integer or real number as a value actually
restrict the value to some range, often to a non-negative value.
4.3.2 Lengths
Lengths refer to horizontal or vertical measurements.
The format of a length value (denoted by in this specification) is an
optional sign character (’+’ or ’-’, with ’+’ being the default) immediately followed
by a (with or without a decimal point) immediately followed by a unit
identifier (e.g., px, deg, etc.). After the ’0’ length, the unit identifier is optional.
Some properties allow negative length values, but this may complicate the
formatting model and there may be implementation-specific limits. If a negative
length value cannot be supported, it should be converted to the nearest value
that can be supported.
There are two types of length units: relative and absolute. Relative length units
specify a length relative to another length property. Style sheets that use relative
units will more easily scale from one medium to another (e.g., from a computer
display to a laser printer).
Relative units are:
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