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Table of Contents
Common uses and usage
1. Picture fade effect
2. Background mask layer
3. Control the transparency changes of icons or buttons
Things to pay attention to when using
Tips: Progressive display hide
Home Web Front-end CSS Tutorial Describe the `opacity` property

Describe the `opacity` property

Jul 15, 2025 am 01:23 AM
css Opacity

opacity is an attribute in CSS that controls the overall transparency of an element, ranging from 0 (fully transparent) to 1 (fully opaque). 1. It is often used for the image hover fade effect, and enhances the interactive experience by setting the opacity transition; 2. Making a background mask layer to improve text readability; 3. Visual feedback of the control buttons or icons when disabled. Note that it affects all child elements, and unlike rgba, the latter affects only the specified color part. Smooth animations can be achieved with transition, but frequent use may affect performance. It is recommended to use them in combination with will-change or transform. Rational application of opacity can enhance the hierarchy and interactivity of the page, but it should avoid interfering with user operations and reading experience.

Describe the `opacity` property

One of the most common ways to set transparency is to use the opacity property. It can control the degree of opacity of an element, from completely transparent to completely imperceptible to name, and is suitable for multiple scenes such as pictures, text, and backgrounds.

Describe the `opacity` property

What is opacity

opacity is an attribute in CSS that sets the overall transparency of an element. Its value range is 0 to 1 :

  • 0 means complete transparency (invisible)
  • 1 means total opaque (default)
  • The value in the middle, such as 0.5 , means translucent

It should be noted that opacity acts on the entire element and all its child elements. That is to say, if you set opacity: 0.5 for a container, then everything inside will become transparent together.

Describe the `opacity` property

Common uses and usage

1. Picture fade effect

In web design, we often see some images becoming more transparent when hovering, which is achieved through opacity :

 img {
  opacity: 1;
  transition: opacity 0.3s;
}

img:hover {
  opacity: 0.7;
}

This allows users to feel interactive feedback while maintaining a soft visual transition.

Describe the `opacity` property

2. Background mask layer

Sometimes we will cover the picture with a translucent black or white layer to make the text clearer. for example:

 <div class="overlay"></div>
 .overlay {
  opacity: 0.5;
  background-color: black;
}

This method is often used in the text background of the banner area to improve readability.

3. Control the transparency changes of icons or buttons

When the button or icon is disabled, opacity is usually added to indicate that it is not clickable:

 .button.disabled {
  opacity: 0.4;
  pointer-events: none;
}

This not only visually prompts the user to be inoperable, but also cooperates with pointer-events to prevent interaction.

Things to pay attention to when using

  • Will affect child elements : As mentioned earlier, opacity is inherited. After the parent element has set transparency, all content inside will also be affected.
  • Performance issues : Frequent changes opacity in animations may cause redrawing and re-arrangement. Although modern browsers are better optimized, it is still recommended to use it with will-change or transform .
  • Differences from rgba :
    • opacity affects the entire element
    • rgba() only affects the specified color part and will not make the text, etc. transparent

For example:

 background-color: rgba(0, 0, 0, 0.5); /* only affects the background*/
color: rgba(255, 0, 0, 0.5); /* only affects text color*/

Tips: Progressive display hide

Combined with transition , we can achieve smooth display/hide animation:

 .fade-box {
  opacity: 0;
  transition: opacity 0.4s ease;
}

.fade-box.show {
  opacity: 1;
}

Then, control the class name switching through JavaScript to achieve a fading effect.

Basically that's it. Rational use of opacity can make the page more layered and interactive, but it is not advisable to abuse it, especially be careful not to affect the user's reading or operation experience.

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