Basic Attributes and Styles

Lengths refer to distance measurements.

The format of a length value (denoted by <length> in this specification) is a <number> (with or without a decimal point) immediately followed by a unit identifier (e.g., px, em, etc.). After a zero length, the unit identifier is optional.

Relative units are:

• em: the 'font-size' of the relevant font
• ex: the 'x-height' of the relevant font

Absolute length units are fixed in relation to each other. They are mainly useful when the output environment is known. The absolute units consist of the physical units (in, cm, mm, pt, pc) and the px unit:

• in: inches — 1in is equal to 2.54cm.
• cm: centimeters
• mm: millimeters
• pt: points — the points used by CSS are equal to 1/72nd of 1in.
• pc: picas — 1pc is equal to 12pt.
• px: pixel units — 1px is equal to 0.75pt.

The format of an RGB value in hexadecimal notation is a '#' immediately followed by either three or six hexadecimal characters. The three-digit RGB notation (#rgb) is converted into six-digit form (#rrggbb) by replicating digits, not by adding zeros. For example, #fb0 expands to #ffbb00. This ensures that white (#ffffff) can be specified with the short notation (#fff) and removes any dependencies on the color depth of the display.

The format of an RGB value in the functional notation is 'rgb(' followed by a comma-separated list of three numerical values (either three integer values or three percentage values) followed by ')'. The integer value 255 corresponds to 100%, and to F or FF in the hexadecimal notation: rgb(255,255,255) = rgb(100%,100%,100%) = #FFF. White space characters are allowed around the numerical values.

All RGB colors are specified in the sRGB color space (see [SRGB]). User agents may vary in the fidelity with which they represent these colors, but using sRGB provides an unambiguous and objectively measurable definition of what the color should be, which can be related to international standards (see [COLORIMETRY]).

There are two types of font family names:

<family-name>

The name of a font family of choice. In the last example, "Gill" and "Helvetica" are font families.

<generic-family>

In the example above, the last value is a generic family name. The following generic families are defined:

• 'serif' (e.g., Times)
• 'sans-serif' (e.g., Helvetica)
• 'cursive' (e.g., Zapf-Chancery)
• 'fantasy' (e.g., Western)
• 'monospace' (e.g., Courier)

Style sheet designers are encouraged to offer a generic font family as a last alternative. Generic font family names are keywords and must NOT be quoted.

Font family names must either be given quoted as strings, or unquoted as a sequence of one or more identifiers. This means most punctuation characters and digits at the start of each token must be escaped in unquoted font family names.