Transitions

Transitions add motion and change to the page:

tell the browser to ease one value into another when the value changes. Think state changes on a link
transition-* properties add motion to the page.

Accessibility

Some users set up their OSs to prevent certain motions on the screen. Check whether these settings exist with the prefers-reduced-motion media query as part of your reset styles:

@layer reset {
    @media (prefers-reduced-motion: reduce) {
        *,
        *::before,
        *::after {
            animation-duration: 0.01ms !important;
            animation-iteration-count: 1 !important;
            transition-duration: 0.01ms !important;
            scroll-behavior: auto !important;
        }
    }
}

Basics

Transitions morph styles from a first ruleset to a second ruleset, where the second ruleset is defines styles for a state change on an element.

Add the transition-* properties to the rulese that targets the element at all times. This is usually the ruleset for the element’s original (static) state:
- transition-property to define which property you want to transition after a state change. Can be a specific property, or all for all properties defined on the state.
- transition-duration is the amount of time it takes to transition into the properties for the new state. Takes s or ms.
On the new state’s ruleset, define the properties that you want to change.

Example

When you hover over this button, the border radius and background color change to the styles defined in the :hover state ruleset. Border radius changes even though it is not defined on the static element ruleset:

button {
  padding: 0.3em 1em;
  border: 0;
  font-size: 1rem;
  color: white;
  background-color: oklch(74% 0.11 195deg);
  transition-property: all;
  transition-duration: 0.5s;
}

button:hover {
  border-radius: 1em;
  background-color: oklch(55% 0.16 24deg);
}

transition

Not all properties can be animated (apply a transition to it). For example, you can’t animate the display property:

Look in the MDN docs for details about whether you can animate the property. If it has an animation type of discrete, it cannot be animated.
In general, properties that accept a length, number, color, or calc() function can be animated
When you create a transition, slow it down to 2 or 3 seconds to make sure it is behaving the way you want it to.

transition property is shorthand for these values:

transition: <affected-property> <duration> <timing-function> <delay>
/* for example */
transition: background-color 0.3s linear 0.5s;

/* target multiple properties */
transition: background-color, color, font-size 0.3s linear 0.5s;

/* different transitions to different properties */
transition: border-radius 0.3s linear, background-color 0.6s ease;
/* equivalent in longhand */
transition-property: border-radius, background-color;
transition-duration: 0.3s, 0.6s;
transition-timing-function: linear, ease;

affected property is the property that you want to change
duration is how long the transition takes. It is a time value expressed in s or ms, cannot be 0.
- For hover effects, use a transition duration between 200-500ms or it will seem like your website is slow.
timing-function controls the rate of change between the transition.
delay lets you specify a time value before the transition begins

Timing functions

The timing function defines how the property value transitions from one value to another:

does it change at a steady speed or start slowly and accelerate?

See MDN docs for detailed descriptions.

Possible values:

linear: changes at a constant rate
ease: increases in velocity until the middle, then slows down
ease-in: starts slow but then accelerates until transition completes
ease-out: starts quickly but then decelerates until transition completes
ease-in-out: starts slowly, accelerates, then decelerates until complete

Use cases

Linear: Color changes and fade in/out effects
Decelerating: User-initiated changes. Use a flavor of ease-out. This lets them see a response to their actions quickly
Accelerating: System-initiated changes. Use a flavor of ease-in. This draws the user’s attention at first and then speeds up to complete the transition.

cubic-bezier()

Create a custom timing function with cubic-bezier() function. Timing functions are based on Bezier curves, which calculate a property’s value as a function of change over time.

You can experiment with timing functions in the DevTools pane:

Go to the element with the transition property.
Select the box with the curved line to open the cubic bezier editor.

You can also experiment at cubic-bezier.com.

step()

Steps are not very practical, but here are some ideas on CSS Tricks.

Instead of a fluid transition, you can transition in discrete steps. This takes two parameters: the number of steps, and the start or end keyword that indicates whether the change should occur at the start or end of each step:

.box {
  position: absolute;
  left: 0;
  height: 50px;
  width: 50px;
  background-color: oklch(70% 0.18 145deg);
  transition: all 1s steps(3);
}

Examples

This example is a menu that opens when you click the menu button. It has these transitions:

The colors on the menu button and list item change when you hover the mouse on them
menu fades in and out when you click the toggle button
- transition opacity from 0 to 1
- remove the menu drawer from the page with visibility, which is animatable–unlike display. visibility accepts either visible or hidden. If an element is hidden, it is still in the document flow, but this doesnt matter with the menu we are creating because it is absolutely positioned.
set the transition to transition: opacity 0.2s linear, visibility 0s linear 0.2s;
- When the menu closes, you transition the opacity for 0.2s, and then transitions the visibility in 0s, but after a 0.2s delay. This delay is enough time for the opacity to fade out, and then the visiblity completely removes it afterwards.
- When the menu opens, you set the visibility to visible, and remove the delay by setting transition-delay: 0s

<div class="dropdown" aria-haspopup="true">                             <!-- dropdown container -->
      <button class="dropdown__toggle" type="button">Menu</button>      <!-- dropdown button -->
      <div class="dropdown__drawer">                                    <!-- container for menu lis-->
        <ul class="menu" role="menu">                                   <!-- menu items -->
          <li role="menuitem">
            <a href="/features">Features</a>
          </li>
          <li role="menuitem">
            <a href="/pricing">Pricing</a>
          </li>
          <li role="menuitem">
            <a href="/support">Support</a>
          </li>
          <li role="menuitem">
            <a href="/about">About</a>
          </li>
        </ul>
      </div>
    </div>
    <p><a href="/read-more">Read more</a></p>
    <script>
        let toggle = document.getElementsByClassName("dropdown__toggle")[0];    // toggle .is-open class on click
        let dropdown = toggle.parentElement;
        toggle.addEventListener("click", function (e) {
            dropdown.classList.toggle("is-open");
        });
    </script>

@layer modules {
  .dropdown {
    --border-color: oklch(61% 0.08 314deg);
    --text-color: oklch(39% 0.06 314deg);
    --text-color-focused: oklch(39% 0.2 314deg);
    --background-color: white;
    --highlight-color: oklch(95% 0.01 314deg);
  }
  .dropdown__toggle {
    display: block;
    padding: 0.5em 1em;
    border: 1px solid var(--border-color);
    color: var(--text-color);
    background-color: var(--background-color);
    font: inherit;
    text-decoration: none;
    transition: background-color 0.2s linear;       /* transition color on button when state changes */
  }
  .dropdown__toggle:hover {
    background-color: var(--highlight-color);       /* color button transitions to on hover */
  }
  .dropdown__drawer {
    position: absolute;
    background-color: var(--background-color);
    width: 10em;
    visibility: hidden;                                             /* hidden by default */
    opacity: 0;                                                     /* completely opaque by default */
    transition: opacity 0.2s linear, visibility 0s linear 0.2s;     /* opacity transition in 0.2s */
  }                                                                 /* delay visibility transition for 0.2s */
  .dropdown.is-open .dropdown__drawer {
    visibility: visible;
    opacity: 1;
    transition-delay: 0s;                                           /* remove transition delay so its immediately visible */
  }

  .menu {
    padding-left: 0;
    margin: 0;
    list-style: none;
  }
  .menu > li + li > a {
    border-top: 0;
  }

  .menu > li > a {
    display: block;
    padding: 0.5em 1em;
    color: var(--text-color);
    background-color: var(--background-color);
    text-decoration: none;
    transition: all 0.2s linear;                    /* transition menu item color on state change */
    border: 1px solid var(--border-color);
  }

  .menu > li > a:hover {
    background-color: var(--highlight-color);       /* menu item background-color transitions on hover */
    color: var(--text-color-focused);               /* menu item color transitions on hover */
  }
}

This menu needs to transition the menu height from 0 to auto, but you can’t transition from an explicit length (0) to auto. You need JS to figure out what the height should be with the menu’s scrollHeight property.

This example reuses the HTML/CSS from the previous section with updated JS:

let toggle = document.getElementsByClassName("dropdown__toggle")[0];
let dropdown = toggle.parentElement;
let drawer = document.getElementsByClassName('dropdown__drawer')[0];
// get the scrollHeight
let height = drawer.scrollHeight;

// if the is-open class is present, set the element height to the 
// scrollHeight value. Otherwise, the height is 0.
toggle.addEventListener("click", function (e) {
    dropdown.classList.toggle("is-open");
    if (dropdown.classList.contains('is-open')) {
        drawer.style.setProperty('height', height + 'px');
    } else {
        drawer.style.setProperty('height', '0');
    }
});