Table of contents

Appendices

5.1 Specified, Computed, and Actual Values, and Inheritance

For every property that is applicable to a given formatting object, it is necessary to determine the value of the property. Three variants of the property value are distinguished: the specified value, the computed value, and the actual value. The "specified value" is one that is placed on the formatting object during the tree-construction process. A specified value may not be in a form that is directly usable; for example, it may be a percentage or other expression that must be converted into an absolute value. A value resulting from such a conversion is called the "computed value". Finally, the computed value may not be realizable on the output medium and may need to be adjusted prior to use in rendering. For example, a line width may be adjusted to become an integral number of output medium pixels. This adjusted value is the "actual value."

Specified Values[top]

The specified value of a property is determined using the following mechanisms (in order of precedence):

1. If the tree-construction process placed the property on the formatting object, use the value of that property as the specified value. This is called "explicit specification".

2. Otherwise, if the property is inheritable, use the value of that property from the parent formatting object, generally the computed value (see below).

3. Otherwise use the property's initial value, if it has one. The initial value of each property is indicated in the property's definition. If there is no initial value, that property is not specified on the formatting object. In general, this is an error.

Since it has no parent, the root of the result tree cannot use values from its parent formatting object; in this case, the initial value is used if necessary.

Computed Values[top]

Specified values may be absolute (i.e., they are not specified relative to another value, as in "red" or "2mm") or relative (i.e., they are specified relative to another value, as in "auto", "2em", and "12%"), or they may be expressions. For most absolute values, no computation is needed to find the computed value. Relative values, on the other hand, must be transformed into computed values: percentages must be multiplied by a reference value (each property defines which value that is), values with a relative unit (em) must be made absolute by multiplying with the appropriate font size, "auto" values must be computed by the formulas given with each property, certain property values ("smaller", "bolder") must be replaced according to their definitions. The computed value of any property that controls a border width where the style of the border is "none" is forced to be "0pt".

Some properties have more than one way in which the property value can be specified. The simplest example of such properties are those which can be specified either in terms of a direction relative to the writing-mode (e.g., padding-before) or a direction in terms of the absolute geometric orientation of the viewport (e.g., padding-top). These two properties are called the relative property and the absolute property, respectively. Collectively, they are called "corresponding properties".

Specifying a value for one property determines both a computed value for the specified property and a computed value for the corresponding property. Which relative property corresponds to which absolute property depends on the writing-mode. For example, if the "writing-mode" at the top level of a document is "lr-tb", then "padding-start" corresponds to "padding-left", but if the "writing-mode" is "rl-tb", then "padding-start" corresponds to "padding-right". The exact specification of how to compute the values of corresponding properties is given in [compcorr] .

In most cases, elements inherit computed values. However, there are some properties whose specified value may be inherited (e.g., some values for the "line-height" property). In the cases where child elements do not inherit the computed value, this is described in the property definition.

Actual Values[top]

A computed value is in principle ready to be used, but a user agent may not be able to make use of the value in a given environment. For example, a user agent may only be able to render borders with integer pixel widths and may, therefore, have to adjust the computed width to an integral number of media pixels. The actual value is the computed value after any such adjustments have been applied.

Inheritance[top]

Some of the properties applicable to formatting objects are "inheritable." Such properties are so identified in the property description. The inheritable properties can be placed on any formatting object. The inheritable properties are propagated down the formatting object tree from a parent to each child. (These properties are given their initial value at the root of the result tree.) For a given inheritable property, if that property is present on a child, then that value of the property is used for that child (and its descendants until explicitly re-set in a lower descendant); otherwise, the specified value of that property on the child is the computed value of that property on the parent formatting object. Hence there is always a specified value defined for every inheritable property for every formatting object.