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used value

The used value of a CSS property is the final value of that property after all calculations have been performed.

After the user agent has finished its calculations, every CSS property has a used value. The used values of dimensions (e.g., width, line-height) are in pixels. The used values of shorthand properties (e.g., background) are consistent with those of their component properties (e.g., background-color, display) and with position and float.

For some properties, JavaScript can retrieve the used value through the window.getComputedStyle method.

Details

There are four 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 span with position: absolute will have its computed display changed to block). Then, 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.

Finally, transformed according to the limitations of the local environment, the result is actual value. The actual value is the used value after any approximations have been applied. For example, a user agent may only be able to render borders with integer pixel widths, and therefore have to approximate the computed width, or the user agent may be forced to use only black and white shades, instead of full color. These steps are calculated internally.

JavaScript can read only the final used values with window.getComputedStyle. This method may instead return computed values, depending on the property. The values it returns are generically called resolved values).

Example

Compute and show the used width of three elements (updates on resize):

HTML

<div id="no-width">
  <p>No explicit width.</p>
  <p class="show-used-width">..</p>

  <div id="width-50">
    <p>Explicit width: 50%.</p>
    <p class="show-used-width">..</p>

    <div id="width-inherit">
      <p>Explicit width: inherit.</p>
      <p class="show-used-width">..</p>
    </div>
  </div>
</div>

CSS

#no-width {
  width: auto;
}

#width-50 {
  width: 50%;
}

#width-inherit {
  width: inherit;
}

/* Make results easier to see: */
div {
  border: 1px solid red;
  padding: 8px;
}

JavaScript

function updateUsedWidth(id) {
  var div = document.getElementById(id);
  var par = document.querySelector(`#${id} .show-used-width`);
  var wid = window.getComputedStyle(div)["width"];
  par.textContent = `Used width: ${wid}.`;
}

function updateAllUsedWidths() {
  updateUsedWidth("no-width");
  updateUsedWidth("width-50");
  updateUsedWidth("width-inherit");
}

updateAllUsedWidths();
window.addEventListener('resize', updateAllUsedWidths);

Result

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. An element could then 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 CSS 2.1 properties that do depend on layout, so they have a different computed value and used value: (taken from CSS 2.1 Changes: Specified, computed, and actual values):

  • background-position
  • 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
  • text-indent

Specification

CSS Level 2: Used Values

See also

© 2005–2017 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/CSS/used_value