6.1.2 Computed values
Specified values are resolved to computed values during the cascade; for example URIs are made absolute and 'em' and 'ex' units are computed to pixel or absolute lengths. Computing a value never requires the user agent to render the document.
6.1.3 Used values
Computed values are processed as far as possible without formatting the document. Some values, however, can only be determined when the document is being laid out. For example, if the width of an element is set to be a certain percentage of its containing block, the width cannot be determined until the width of the containing block has been determined. The used value is the result of taking the computed value and resolving any remaining dependencies into an absolute value.
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Committed r239608: <https://trac.webkit.org/changeset/239608>