Computed Values Patterns

Computed Value (the concept in the propdef table) is a UA-internal representation of a property mainly meant for supporting animations. It’s necessary for animatable properties to produce interpolable computed values. Inferred values (such as with various <position> variants, or single-value border-radius-* values) will likely need to be added to the computed value to produce an interpolable result. Even if a property is not animatable, it’s probably useful to design the computed value such that it could support interpolation.

Serialization (such as the result from getComputedStyle) is a valid CSS string representation that might only have a tenuous connection to the computed value representation. Its main characteristic is that it must ‘round-trip’ with parsing. Inferred values can be omitted here. If a grammar allows ordering options, the serialized value should prefer an order (usually what’s presented in the grammar).

Here are some examples using background-position: 

Declared value: right 10px
Computed value: (100% - 10px), (50% + 0px)
getComputedValue result: right 10px

Declared value: bottom 50px top 10%
Computed value: (10% + 0px), (100% - 50px)
getComputedValue result: 10% bottom 50px

Declared value: center center
Computed value: (50% + 0px), (50% + 0px)
getComputedValue result: center

This page lists a number of design patterns that have been identified for computed values.

If you're a spec editor, you should check the “computed value:” definition of the properties in your spec with this document.

  1. computed values should depend only on the specified or computed values of properties on the element or its parent
  2. computed values must never depend on layout
  3. shorthand properties do not have computed values. For them, simply specify:
    • Computed value: see individual properties
  4. url() resolvability cannot affect the computed value, since the concept of computed value shouldn't require accessing the network.
  5. which format a url() resource is (e.g. whether the browser supports it) also cannot affect the computed value
  6. URIs in computed values are absolute. E.g.
    • Computed value: as specified, except with any relative URLs converted to absolute
  7. properties that just accept keyword or IDREF values, should just specify:
    • Computed value: as specified
  8. (disputed) computed values should never depend on containing block hierarchy (though note that they already do in CSS21, eg 'height' for percentage values (http://lists.w3.org/Archives/Public/www-style/2011Sep/0008.html)