How to change the CSS background opacity of an element

Matthew C.

—March 15, 2023

The Problem

You want to change the background opacity of an element. For example, you may want to decrease the opacity of a background image to make any text on the image more readable. The opacity CSS property sets the opacity of an element. If you decrease the opacity of the background of an element, the opacity of the child elements will be decreased as well. How do you decrease the opacity of the background without affecting the opacity of child elements?

The Solution

The opacity of a parent element is not inherited by child elements. Setting the opacity of an element’s background to a value below 1 adds the element to a new stacking context. The opacity of the parent’s background sets the maximum perceived opacity of the child elements. The child elements will have an opacity relative to the parents’ background opacity.

Here are three solutions that prevent the child element from having its opacity reduced when the opacity of the parent element is reduced. The first solution is for parent elements with a background color, the second and third are for parent elements that have an image as a background.

Setting the alpha channel value of the background color

Set the background-color property with a color value that has an alpha channel that allows you to change the transparency of the color.

.parent {
  background-color: rgba(0, 0, 0, 0.1);
}

You can also set the background property, which is a shorthand that allows you to set all of the background CSS properties at once.

Using a CSS pseudo-element and absolute positioning for a background image

You can use a ::before or ::after CSS pseudo-element that’s absolutely positioned relative to the parent element. You can add a background image to the pseudo-element and change its opacity:

.parent {
  position: relative;
}

.parent::before {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  opacity: 0.1;
  background: url(https://source.unsplash.com/random);
  z-index: -1;
}

The ::before pseudo-element is positioned relative to the parent element. We make it take up the full space of the parent element. We set the z-index of the ::before pseudo-element to -1 so that the background image is positioned behind any of the parent’s content. You can change the positioning and styling of the background image using background properties such as background-position and background-size.

Using an image element and absolute positioning for a background image

If you want to use an image from an <img> element as the background image, you can also make use of absolute positioning to make the image cover the background of the parent and set its opacity without affecting the child elements’ opacity:

<div class="parent">
  <img
    class="img-bg"
    src="https://source.unsplash.com/random"
    alt="Random image">
  <div>
    text
  </div>
</div>

Set the width and height of the image to 100% so that it covers the parent:

.parent {
  position: relative;
  width: 350px;
  height: 100px;
}

.img-bg {
  opacity: 0.1;
  position: absolute;
  width: 100%;
  height: 100%;
  object-fit: cover;
}

The object-fit property allows you to determine how the image should be resized to fit its container, the parent element.