opacity

<alpha-value>

A <number> in the range 0.0 to 1.0, inclusive, or a <percentage> in the range 0% to 100%, inclusive, representing the opacity of the channel (that is, the value of its alpha channel). Any value outside the interval, though valid, is clamped to the nearest limit in the range.

Value	Meaning
0	The element is fully transparent (that is, invisible).
Any <number> strictly between 0 and 1	The element is translucent (that is, content behind the element can be seen).
1 (default value)	The element is fully opaque (visually solid).

opacity applies to the element as a whole, including its contents, even though the value is not inherited by child elements. Thus, the element and its children all have the same opacity relative to the element's background, even if they have different opacities relative to one another.

To change the opacity of a background only, use the background property with a color value that allows for an alpha channel. For example:

background: rgb(0 0 0 / 40%);

When opacity value is set to 0, the element and all of its children appear invisible, but they are still part of the DOM. That means they still register pointer events and, if the elements are in a tabbing order, they do get focus. For good usability, make sure to make such elements visible when they receive user interactions or use the CSS pointer-events property to disable pointer events and take the element out of the tab order by disabling with the disabled attribute or setting tab-index="-1" for non-form-related interactive elements.

Using opacity with a value other than 1 places the element in a new stacking context.

Opacity alone should not be used to provide information to screen readers. Use the HTML hidden attribute, CSS visibility, or CSS display style properties. It's best to avoid using aria-hidden attribute, but if the element is hidden with opacity, then hide it from screen readers as well.

When transitioning the opacity of elements as you add them to the page when content was formerly hidden with visibility: hidden, display: none, or content-visibility: hidden, you need to include both a @starting-style and transition-behaviour: allow-discrete:

.card {
  transition:
    opacity 5s,
    display 5s;
  background-color: orange;

  transition-behavior: allow-discrete;
  @starting-style {
    opacity: 0;
  }
}

.card.hidden {
  display: none;
  opacity: 0;
}

To enable first-style transitions, @starting-style rules are needed. In the above code, setting opacity: 0 in @starting-style provides a starting point for the transition when the element receives its initial style update. For more details, see @starting-style.

Setting transition-behavior: allow-discrete is required to transition to display: none. See the transition-behavior property for more details.

If text opacity is adjusted, it is important to ensure that the contrast ratio between the color of the text and the background the text is placed over is high enough that people experiencing low vision conditions will be able to read the content of the page.

Color contrast ratio is determined by comparing the luminosity of the opacity-adjusted text and background color values. In order to meet current Web Content Accessibility Guidelines (WCAG), a ratio of 4.5:1 is required for text content and 3:1 for larger text such as headings. Large text is defined as 18.66px and bold or larger, or 24px or larger.

• WebAIM: Color Contrast Checker
• MDN Understanding WCAG, Guideline 1.4 explanations
• Understanding Success Criterion 1.4.3 | W3C Understanding WCAG 2.0

Various operating systems provide a preference for reducing transparency. To set the opacity based on the user's operating systems transparency preferences, use the prefers-reduced-transparency media query.

opacity = 
<opacity-value>  

<opacity-value> = 
<number>      |
<percentage>  

The following example demonstrates how the opacity property changes the opacity of the entire element and content, thus making the text very hard to read.

HTML

<div class="light">You can barely see this.</div>
<div class="medium">This is easier to see.</div>
<div class="heavy">This is very easy to see.</div>

CSS

div {
  background-color: yellow;
  font-weight: bold;
  font-size: 130%;
}
.light {
  opacity: 0.2; /* Barely see the text over the background */
}
.medium {
  opacity: 0.5; /* See the text more clearly over the background */
}
.heavy {
  opacity: 0.9; /* See the text very clearly over the background */
}

Result

In the following example opacity is changed on hover, so the striped background image on the parent element shows through the image.

HTML

<div class="wrapper">
  <img
    src="//interactive-examples.mdn.mozilla.net/media/dino.svg"
    alt="MDN Dino"
    width="128"
    height="146"
    class="opacity" />
</div>

CSS

img.opacity {
  opacity: 1;
}

img.opacity:hover {
  opacity: 0.5;
}

.wrapper {
  width: 200px;
  height: 160px;
  background-color: #f03cc3;
  background-image: linear-gradient(
    90deg,
    transparent 50%,
    rgb(255 255 255 / 50%) 50%
  );
  background-size: 20px 20px;
}

Result

To style elements based on user's operating systems transparency preferences, use the prefers-reduced-transparency media query. The following example demonstrates how to use the prefers-color-scheme media query to specify the desired opacity based on the user's preferences.

.element {
  opacity: 0.5;
}

@media (prefers-reduced-transparency) {
  .element {
    opacity: 1;
  }
}

• prefers-reduced-transparency media query
• CSS color module