<easing-function>

linear

Specifies a constant rate of interpolation, with no change in the rate of progress throughout the duration (that is, no acceleration or deceleration). This keyword value is equivalent to both cubic-bezier(0, 0, 1, 1) and linear(0, 1) functions.

<linear-easing-function>

Specifies a linear() function with one or more comma-separated linear stops, each containing up to two optional stop lengths, to control the progress of an animation or transition.

The linear() function specifies a <linear-stop-list>, a comma-separated list of points along the animation or transition progress. Each point or <linear-stop> in the list is specified as a <number> between 0 and 1. By default, each stop in the <linear-stop-list> is equidistant. To have more control on the progress of the animation or transition, each point can include up to two optional <percentage> <linear-stop-length> values.

• <number>: Represents a point in time along the duration of the animation or transition. The value 0 represents the start of the iteration and 1 represents the end. Values outside the 0 to 1 range are also allowed.
• <percentage>: Indicates the position of a linear stop along the duration. It can take up to two values. If one value is specified, it defines the start of the associated linear stop. If two percentage values are specified, they define the length of the stop: the first percentage indicates the starting point and the second percentage indicates the ending point for that segment in the animation or transition. If no <percentage> value is specified, which is the default, the stops are spread evenly along the timeline.

<cubic-bezier-easing-function>

Specifies a Bézier curve to shape the progress of an animation or a transition. In CSS, Bézier curves are defined by four control points that mathematically describe the curve: a starting point, an ending point, and two control points. The cubic Bézier easing function can be defined in one of these two ways: by creating a custom curve with a four-parameter cubic-bezier() function call or by using one of the predefined keyword values, which map to the commonly used Bézier curve parameters. The predefined keyword values include:

ease: This keyword represents the easing function cubic-bezier(0.25, 0.1, 0.25, 1). It indicates that the interpolation starts slowly, accelerates sharply, and then slows gradually towards the end. It is similar to the ease-in-out keyword, though it accelerates more sharply at the beginning.

ease-in: This keyword represents the easing function cubic-bezier(0.42, 0, 1, 1). It indicates that the interpolation starts slowly, then progressively speeds up until the end, at which point it stops abruptly.

ease-out: This keyword represents the easing function cubic-bezier(0, 0, 0.58, 1). It indicates that the interpolation starts abruptly and then progressively slows down towards the end.

ease-in-out: This keyword represents the easing function cubic-bezier(0.42, 0, 0.58, 1). It indicates that the interpolation starts slowly, speeds up, and then slows down towards the end. At the beginning, it behaves like the ease-in keyword; at the end, it is like the ease-out keyword.

cubic-bezier(): This function accepts four <number> values to shape a curve.

• <number>: Specifies the location of P1 and P2 points on the curve. <x1> and <y1> are the coordinates for point P1, and <x2> and <y2> are the coordinates for point P2. <x1> and <x2> values must be between 0 and 1, otherwise the function will not work as expected.

<step-easing-function>

Specifies a steps() function that divides the animation into a set number of equal-length intervals or "steps", causing the animation to jump from one step to the next rather than transitioning smoothly. This parameter accepts one of the following two keyword values that map to predefined steps() functions or a custom steps() function:

step-start: This keyword represents the easing function steps(1, jump-start) or steps(1, start). It indicates that the interpolation jumps immediately to its final state, where it stays until the end.

step-end: This keyword represents the easing function steps(1, jump-end) or steps(1, end). It indicates that the interpolation stays in its initial state until the end, at which point it jumps directly to its final state.

steps(): This function accepts a positive <integer> and an optional <step-position>.

• <integer>: Represents the number of equidistant intervals or 'steps'. It must be a positive integer greater than 0 unless the second parameter is jump-none, in which case, it must be a positive integer greater than 1.
• <step-position>: Specifies the timing of the jump to occur either at the start, at the end, at both start and end, or at neither. The possible keyword values include:
  • jump-start denotes that the first jump occurs right at the start, essentially at the 0 point. No time is spent at the 0% mark.
  • jump-end denotes that the last jump occurs right at the end, essentially at the 1 point. No time is spent at the 100% mark. This is the default value when no <step-position> is specified.
  • jump-none denotes that no jump occurs either at the beginning or at the end, effectively removing a step during the duration. Instead, the progress holds steady at both the 0% mark and 100% marks. The duration of these holds is determined by dividing the total duration by the number of steps (1/n).
  • jump-both denotes that jumps occur at both the beginning and the end, essentially occurring at both the 0 and 1. Effectively, a step is added at both the ends. No time is spent at the 0% and 100% marks.
  • start is the equivalent of jump-start.
  • end is the equivalent of jump-end.

There are three types of easing functions:

• Linear
• Cubic Bézier
• Steps

The linear() function creates a piecewise linear easing, allowing the approximation of complex animations and transitions by interpolating linearly between the specified points. The interpolation occurs at a constant rate from beginning to end. A typical use of the linear() function is to provide many points to approximate any curve.

For example, the linear(0, 0.25, 1) function has linear stops of 0, 0.25, and 1 as arguments. The animation or transition starts at point 0, moves linearly to 0.25, and then continues linearly to point 1. Since no <linear-stop-length> percentage is specified, the same duration (50%) is used to go from 0 to 0.25 and from 0.25 to 1.

By default, the stops are equidistant. So if there are 5 stops, they will be at 0%, 25%, 50%, 75%, and 100% of the duration. You can use optional <linear-stop-length> percentage values to provide finer control, defining the start and/or length of each segment, allowing for a more controlled progression of the transition.

Consider an animation with a duration time of 100 seconds and a change of 100 pixels. Let's look at an example where the easing of the animation is specified: linear(0, 0.25 75%, 1). In this case, the animation progresses to 25 pixels (25% of its total change) in the first 75 seconds (75% of the duration). The last 75 pixels are applied in the remaining 25 seconds of the animation. For the same animation, suppose the easing function is given instead as linear(0, 0.5 25% 75%, 1). Here, the animation reaches 50 pixels (50% of its total change) in 25 seconds (25% of the duration) and remains there for 50 seconds (75% - 25% of the duration). Then the last 50 pixels are applied in the remaining 25 seconds of the duration. Note that linear(0, 0.5 25% 75%, 1) is equivalent to linear(0, 0.5 25%, 0.5 75%, 1).

The cubic-bezier() functional notation defines a cubic Bézier curve. The easing functions in the cubic-bezier subset of easing functions are often called "smooth" easing functions because they can be used to smooth down the start and end of the interpolation. They correlate an input progress to an output progress, both expressed as <number>s. For these values, 0.0 represents the initial state, and 1.0 represents the final state.

A cubic Bézier curve is defined by four points: P0, P1, P2, and P3. The points P0 and P3 represent the start and the end of the curve. In CSS, these points are fixed as the coordinates progress (the abscissa the input progress, the ordinate the output progress). P0 is (0, 0) and represents the initial progress and the initial state. P3 is (1, 1) and represents the final progress and the final state.

Not all cubic Bézier curves are suitable as easing functions because not all are mathematical functions; i.e., curves that for a given abscissa have zero or one value. With P0 and P3 fixed as defined by CSS, a cubic Bézier curve is a function, and is therefore valid, if and only if the abscissas of P1 and P2 are both in the [0, 1] range.

Cubic Bézier curves with the P1 or P2 ordinate outside the [0, 1] range can cause the value to go farther than the final state and then return. In animations, for some properties, such as left or right, this creates a kind of "bouncing" effect.

However, certain properties will restrict the output if it goes outside an allowable range. For example, a color component greater than 255 or smaller than 0 in rgb() will be clipped to the closest allowed value (255 and 0, respectively). Some cubic-bezier() values exhibit this property.

When you specify an invalid cubic Bézier curve, CSS ignores the whole property.

The keywords ease, ease-in, ease-out, and ease-in-out are equivalent to a specific cubic-bezier() value.

The steps() functional notation defines a step function that divides the domain of output values in equidistant steps. This subclass of step functions are sometimes also called staircase functions.

These are a few examples illustrating the steps() function:

steps(2, jump-start) /* Or steps(2, start) */
steps(4, jump-end) /* Or steps(4, end) */
steps(5, jump-none)
steps(3, jump-both)

The keywords step-start and step-end are equivalent to a specific steps() value.

<easing-function> = 
linear                          |
<linear-easing-function>        |
<cubic-bezier-easing-function>  |
<step-easing-function>          

<linear-easing-function> = 
linear( <linear-stop-list> )  

<cubic-bezier-easing-function> = 
ease                                                |
ease-in                                             |
ease-out                                            |
ease-in-out                                         |
cubic-bezier( <number [0,1]> , <number> , <number [0,1]> , <number> )  

<step-easing-function> = 
step-start                             |
step-end                               |
steps( <integer> , <step-position>? )  

<linear-stop-list> = 
[ <linear-stop> ]#  

<step-position> = 
jump-start  |
jump-end    |
jump-none   |
jump-both   |
start       |
end         

<linear-stop> = 
<number>               &&
<linear-stop-length>?  

<linear-stop-length> = 
<percentage>{1,2}  

This example provides an easy comparison between the different easing functions using an animation. From the drop-down menu, you can select an easing function – there are a couple of keywords and some cubic-bezier() and steps() options. After selecting an option, you can start and stop the animation using the provided button.

HTML

<div>
  <div></div>
</div>
<ul>
  <li>
    <button class="animation-button">Start animation</button>
  </li>
  <li>
    <label for="easing-select">Choose an easing function:</label>
    <select id="easing-select">
      <option selected>linear</option>
      <option>linear(0, 0.5 50%, 1)</option>
      <option>ease</option>
      <option>ease-in</option>
      <option>ease-in-out</option>
      <option>ease-out</option>
      <option>cubic-bezier(0.1, -0.6, 0.2, 0)</option>
      <option>cubic-bezier(0, 1.1, 0.8, 4)</option>
      <option>steps(5, end)</option>
      <option>steps(3, start)</option>
      <option>steps(4)</option>
    </select>
  </li>
</ul>

CSS

body > div {
  position: relative;
  height: 100px;
}

div > div {
  position: absolute;
  width: 50px;
  height: 50px;
  background-color: blue;
  background-image: radial-gradient(
    circle at 10px 10px,
    rgb(25 255 255 / 80%),
    rgb(25 255 255 / 40%)
  );
  border-radius: 50%;
  top: 25px;
  animation: 1.5s infinite alternate;
}

@keyframes move-right {
  from {
    left: 10%;
  }

  to {
    left: 90%;
  }
}

li {
  display: flex;
  align-items: center;
  justify-content: center;
  margin-bottom: 20px;
}

JavaScript

const selectElem = document.querySelector("select");
const startBtn = document.querySelector("button");
const divElem = document.querySelector("div > div");

startBtn.addEventListener("click", () => {
  if (startBtn.textContent === "Start animation") {
    divElem.style.animationName = "move-right";
    startBtn.textContent = "Stop animation";
    divElem.style.animationTimingFunction = selectElem.value;
  } else {
    divElem.style.animationName = "unset";
    startBtn.textContent = "Start animation";
  }
});

selectElem.addEventListener("change", () => {
  divElem.style.animationTimingFunction = selectElem.value;
});

Result

These cubic Bézier curves are valid for use in CSS:

/* The canonical Bézier curve with four <number> in the [0,1] range */
cubic-bezier(0.1, 0.7, 1.0, 0.1)

/* Using <integer> is valid because any <integer> is also a <number>. */
cubic-bezier(0, 0, 1, 1)

/* Negative values for ordinates are valid, leading to bouncing effects. */
cubic-bezier(0.1, -0.6, 0.2, 0)

/* Values greater than 1.0 for ordinates are also valid. */
cubic-bezier(0, 1.1, 0.8, 4)

These cubic Bézier curves definitions are invalid:

/* Though the animated output type may be a color,
   Bézier curves work with numerical ratios. */
cubic-bezier(0.1, red, 1.0, green)

/* Abscissas must be in the [0, 1] range or
   the curve is not a function of time. */
cubic-bezier(2.45, 0.6, 4, 0.1)

/* The two points must be defined, there is no default value. */
cubic-bezier(0.3, 2.1)

/* Abscissas must be in the [0, 1] range or
   the curve is not a function of time. */
cubic-bezier(-1.9, 0.3, -0.2, 2.1)

These easing functions are valid:

/* There are 5 treads, the last one happens
   right before the end of the animation. */
steps(5, end)

/* A two-step staircase, the first one happening
   at the start of the animation. */
steps(2, start)

/* The second parameter is optional. */
steps(2)

These easing functions are invalid:

/* The first parameter must be an <integer> and
   cannot be a real value, even if it is equal to one. */
steps(2.0, jump-end)

/* The amount of steps must be non-negative. */
steps(-3, start)

/* There must be at least one step. */
steps(0, jump-none)

• CSS animations
• CSS transitions
• cubic-bezier
• linear() easing generator