The used value of any CSS property is the value of that property after cascading, inheritance, and layout have been calculated. All widths and heights are measured in px. Confusingly, window.getComputedStyle returns used values properties, not the official computed value.
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The used value definition was introduced in CSS 2.1 to be distinct from the computed 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, the computed values and used values are the same. These are the properties that 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