Check out example codes for "css opacity transition". It will help you in understanding the concepts better.

Code Example 1

/* Answer to: "css transition opacity" */

/*
  CSS transitions allows you to change property values smoothly,
  over a given duration.
*/

.myClass {
  vertical-align: top;
  transition: opacity 0.3s; /* Transition should take 0.3s */
  -webkit-transition: opacity 0.3s; /* Transition should take 0.3s */
  opacity: 1; /* Set opacity to 1 */
}

.myClass:hover {
  opacity: 0.5; /* On hover, set opacity to 2 */
}

/* 
  From `opacity: 1;` to `opacity: 0.5;`, the transition time should
  take 0.3 seconds as soon as the client starts to hover over the
  element.
*/

Code Example 2

/*
Opacity transition in CSS

By using this we can add element transition with some delay.
And due to transition delay its look like animation of element.
*/

div {
  transition: opacity 0.5s; /* Transition should take 0.3s */
  -webkit-transition: opacity 0.5s; /* Transition should take 0.3s */
  opacity: 1; /* Set opacity to 1 */
}

/*
I hope it will help you.
Namaste
*/

Code Example 3

div {
  transition: opacity seconds;
}

Code Example 4

/* Answer to: "css transitions" */

div {
  width: 100px;
  height: 100px;
  background: red;
  transition: width 2s;
  /* transition: width 2s, height 4s; */
}
 
div:hover {
  width: 300px;
  height: 1000px;
}

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