css transitions

CSS
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
.btn {
  background-color: #f4511e;
  border: none;
  color: white;
  padding: 16px 32px;
  text-align: center;
  font-size: 16px;
  margin: 4px 2px;
  opacity: 0.6;
  transition: 0.3s;
}

.btn:hover {opacity: 1}
</style>
</head>
<body>

<h2>Fading Buttons - "Fade in Effect"</h2>

<button class="btn">Hover Over Me</button>

</body>
</html>
.selector {
  transition: transform 1s;
}div {
  transition: opacity seconds;
}div {
  transition: width 2s linear 1s; /* Property Duration Easing Delay */
}CSS Transitions
CSS transitions allows you to change property values smoothly, over a given duration.

In this chapter you will learn about the following properties:

transition
transition-delay
transition-duration
transition-property
transition-timing-function

To create a transition effect, you must specify two things:

the CSS property you want to add an effect to
the duration of the effect
Note: If the duration part is not specified, the transition will have no effect, because the default value is 0.

The following example shows a 100px * 100px red <div> element. The <div> element has also specified a transition effect for the width property, with a duration of 2 seconds:

Example
div {
  width: 100px;
  height: 100px;
  background: red;
  transition: width 2s;
}

The transition effect will start when the specified CSS property (width) changes value.

Now, let us specify a new value for the width property when a user mouses over the <div> element:

Example
div:hover {
  width: 300px;
}
Notice that when the cursor mouses out of the element, it will gradually change back to its original style.

Change Several Property Values
The following example adds a transition effect for both the width and height property, with a duration of 2 seconds for the width and 4 seconds for the height:

Example
div {
  transition: width 2s, height 4s;
}/* 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;
}
Source