The used value of any CSS property is the final value of that property after all calculations have been performed. Used values can be retrieved by calling window.getComputedStyle. Dimensions (e.g.
line-height) are all in pixels, shorthand properties (e.g. background) are consistent with their component properties (e.g.
display is consistent with
float, and every CSS property has a value.
There are three steps to calculating any CSS property's final value. First, the specified value is the result of cascading (choosing the most specific stylesheet rule that changes the property), inheritance (using the same computed value as a parent if the property is inheritable), or using the default. Then, the computed value is calculated according to the specification (for example, a
position: absolute will have its computed
display changed to
block). Finally, layout is calculated (dimensions that are
auto or percentages relative to a parent are replaced with pixel values), and the result is the used value. These steps are calculated internally; a script can read only the final used values with window.getComputedStyle.
Difference from computed values
CSS 2.0 defined only computed value as the last step in a property's calculation. Then, CSS 2.1 introduced the distinct definition of used value so that an element could explicitly inherit a width/height of a parent whose computed value is a percentage. For CSS properties that don't depend on layout (e.g. display, font-size, line-height), the computed values and used values are the same. These are the properties that do depend on layout so have a different computed value and used value: (taken from CSS 2.1 Changes: Specified, computed, and actual values):
- bottom, left, right, top
- height, width
- margin-bottom, margin-left, margin-right, margin-top,
- min-height, min-width
- padding-bottom, padding-left, padding-right, padding-top