WDD 231: Web Frontend Development I

CSS Animations and Transitions

Overview

The animation property in CSS enables developers to implement animations without the need for JavaScript. These animations are performance-efficient and are well-suited for creating subtle transitions between defined CSS style states over a specified duration.

The transition property in CSS allows for the smooth interpolation of style changes over a specified duration. By defining which properties should transition and the timing of the transition, developers can enhance user experience through subtle, visually appealing effects triggered by changes in an element's state.

Prepare

The following explains the CSS baseline properties to implement visual effects on web pages.

Keyframes

The @keyframes rule in CSS defines the steps of an animation, specifying how an element's styles should change over time.

You use @keyframes to create an animation by naming it and defining what should happen at different points (called keyframes) during the animation's timeline, such as at 0%, 50%, or 100%.

The following is an example of an animation for a fade in effect:

@keyframes fadeIn {
  0% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}

And, the following example produces a fade color effect that fades from red to yellow:

@keyframes fadeColor {
  from {
    background-color: red;
  }
  to {
    background-color: yellow;
  }
}

Once defined, the new keyframes animation can be referenced in an animation rule as shown below.

Animations

The animation can be defined directly through the animation shorcut property, or by specifying any of its sub-properties directly:

The following applies the fadeIn animation defined above to a box class:

.box {
  animation-name: fadeIn;
  animation-duration: 2s;
}

And the next example applies the fadeColor animation:

.box {
  width: 150px;
  height: 150px;
  background-color: red;

  /* Use the shorthand animation property */
  animation: fadeColor 2s ease-in-out infinite alternate;
}

Transitions

Similar to the animation property, there are a number of rules for transitions that can be defined for each transition sub-property or all at once using the shorthand transition property.

The following transtition properties are available:

The following is an example that smoothly transitions from a white background to a light blue background over 0.5 seconds when the user hovers over the box:

.box {
  width: 200px;
  height: 200px;
  background-color: white;
  border: 1px solid #ccc;

  transition: background-color 0.5s ease;
}

.box:hover {
  background-color: lightblue;
}

In the next example, when the user hovers over the box it will grow in width and height after a slight delay with a smooth motion in and out:

.box {
  width: 100px;
  height: 100px;
  background-color: coral;
  transition: width 0.3s ease-in-out 0.2s, height 0.3s ease-in-out 0.2s;
}

.box:hover {
  width: 150px;
  height: 150px;
}

Transform

The CSS transform property allows you to apply 2D or 3D transformations to an element.

Common transform functions include:

The following is an example of a transform rule that will rotate the element 15 degrees, move it 20 pixels to the right, and enlarge it to 120% of its original size.

.box {
  transform: rotate(15deg) translateX(20px) scale(1.2);
}

Advantages

There are several advantages of using CSS for animations compared to JavaScript or some other means.

Any visual effect should have a clear purpose and developers must consider accessibility best practices, such as ensuring animations can be paused or disabled for users who may experience motion sensitivity or have cognitive disabilities.

Activity Instructions

Build each of the following CSS animations using CodePen or by creating a simple page.

  1. Use the animation shorthand, @keyframes, and transform to create the following "loading ..." animation:
    Check Your Understanding

    This code assumes that there is a div with an id of loader

    #loader {
  margin: 2rem;
  width: 50px;  /* width of the loader */
  height: 50px; /* height of the loader */
  border: 5px solid transparent; /* border width */
  border-color: #999 transparent #999 transparent; /* border colors (4 - 2 transparent) */
  border-radius: 50%; /* make it round */
  animation: spin 1.5s linear infinite; /* spin animation with duration and continuous */
  box-shadow: 0 0 1rem rgb(0 0 255 / 40%); /* blueish shadow effect on round div */
}

#loader::after {
  content: '⚙️';
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  animation: pulse 1.5s ease-in-out infinite;
}

@keyframes spin {
  100% {transform: rotate(360deg);} /* spin 360 degrees */
}

@keyframes pulse { /* pulse the gear emoji */
  0%, 100% {transform: translate(-50%, -50%) scale(1);} /* scale to 1 */
  50% {transform: translate(-50%, -50%) scale(1.25);} /* scale to 1.25 */
}
  2. Now use the transition property to create the following animation which activates when the user hovers over the image.
    Check Your Understanding

    This code assumes that there is a div with an id of loader-t

    .loader-t {
  /* the only thing that changed was using transition versus animation */
  transition: spin 1.5s linear infinite;
}
        
.loader-t::after {
  /* the only thing that changed was using transition versus animation and the size and visibility at start */
  transform: translate(-50%, -50%) scale(0);
  transition: transform 1.5s ease-in-out;
  font-size: 2rem;
 }
        
.loader-t:hover {
  transform: rotate(360deg); /* Rotate on hover */
}
        
.loader-t:hover::after {
  transform: translate(-50%, -50%) scale(1.5); /* Scale on hover */
}

    CSS transition cannot automatically loop like animation.

Test and Share

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Optional Resources

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