Values and Units

There are six main types of values in CSS:

1. keywords (e.g. "pitch-range: inherit")
2. numbers (e.g. "orphans: 3")
3. numbers with unit identifiers (e.g. "border-width: 0.2em")
   or expressed as percentages
4. strings (e.g. "content: 'Figure: '")
5. functions (e.g. "background: url(http://www.w3.org/image)")
6. special cases (e.g. "color: #F00" and "font-size: Helvetica")

Properties are the elements of a declaration that use the values you choose.
Font properties include font-family, font-style, font-variant, font-weight, font-size and line-height, and the 'coverall' font property.
The color and background-color properties are enhanced by defining background-image using background-repeat, background-attachment, and background-position.
Text includes word spacing, letter spacing, text decoration, vertical align, text transform, text align and text indent.
Box properties involve margins, padding, border widths, colors, and also may specify the height & width, float and clear features.

Each CSS property has a formal definition of what types of values it accepts which can be found in the description of the property. Most properties accepts values from several of the above types. Some properties accept space- or comma-separated lists of values.

Keywords

In the formal definition of property values, keywords appear literally. All CSS3 properties accept the keyword value "inherit". Keywords are never quoted.
For example, here is the value definition for the 'border-collapse' property:
Value: collapse | separate | inherit
For example - table { border-collapse: separate }

Numbers

Measurements and positions in Cascading Style Sheets (CSS) properties are indicated in length units. Internet Explorer supports two types of length units: relative and absolute.
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.

Percentages

The format of a percentage value (denoted by <percentage> in this specification) is an optional sign character ('+' or '-', with '+' being the default) immediately followed by a <number> immediately followed by '%'.

Percentage values are always relative to another value, for example a length. Each property that allows percentages also defines the value to which the percentage refers. The value may be that of another property for the same element, a property for an ancestor element, or a value of the formatting context (e.g., the width of a containing block). When a percentage value is set for a property of the root element and the percentage is defined as referring to the inherited value of some property, the resultant value is the percentage times the initial value of that property.

P { font-size: 10pt }
P { line-height: 120% }  /* 120% of 'font-size' */

Strings

Strings can either be written with double quotes or with single quotes. Double quotes cannot occur inside double quotes, unless escaped (as '\"' or as '\22'). Analogously for single quotes ("\'" or "\27").

"this is a 'string'"
"this is a \"string\""
'this is a "string"'
'this is a \'string\''

A string cannot directly contain a newline. To include a newline in a string, use the escape "\A" (hexadecimal A is the line feed character in Unicode, but represents the generic notion of "newline" in CSS).

Functions

Special cases

There are two common cases that fall outside the types described above: color (due to the # notation) and font families (due to missing quotes being legal)

Font families

Font families are like strings, except one may drop the quotes around them.

Colours

The list of keyword color names defined in HTML 4.0, is: aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, purple, red, silver, teal, white, and yellow.

The browser-safe HEX colour codes have pairs of 'digits' in the order red, green, blue.
The three-digit form '#f60' is converted into the six-digit form '#ff6600' by replicating digits.