The <singletransitiontimingfunction>
CSS data type denotes a mathematical function that describes how fast onedimensional values change during transitions or animations. This lets you establish an acceleration curve, so that the speed of the animation can vary over its duration. These functions are often called easing functions.
This function links the time with a ratio of the output value, expressed as a <number>
, with 0.0
representing the initial state, 1.0
the final state.
The output ratio can be greater than 1.0 (or smaller than 0.0). This causes the animation to go farther than the final state, then come back, in a kind of bouncing effect.
Nevertheless, if the output value goes outside of its possible range, such as a component of a color going greater than 255
or smaller than 0
, the value is clipped to its closest allowed value (in the case of a colorcomponent 255
and 0
respectively). Some cubicbezier()
curves exhibit this property.
Values
CSS supports two kinds of timing functions: the subset of the cubic Bézier curves that are functions and staircase functions. The most useful of these functions are given a keyword that allows to easily describe them.
The cubicbezier()
class of timingfunctions
The cubicbezier()
functional notation defines a cubic Bézier curve. As these curves are continuous, they are often used to smooth down the start and end of the animation and are therefore sometimes called easing functions.
A cubic Bézier curve is defined by four points P_{0}, P_{1}, P_{2}, and P_{3}. P_{0} and P_{3} are the start and the end of the curve and, in CSS these points are fixed as the coordinates are ratios (the abscissa the ratio of time, the ordinate the ratio of the output range). P_{0} is (0, 0)
and represents the initial time and the initial state, P_{3} is (1, 1)
and represents the final time and the final state.
Not all cubic Bézier curves are suitable as timing functions as not all are mathematical functions; i.e. curves that for a given abscissa have zero or one value. With P_{0} and P_{3} fixed as defined by CSS, a cubic Bézier curve is a function, and is therefore valid, if and only if the abscissas of P_{1} and P_{2} are both in the [0, 1]
range.
Cubic Bézier curves with the P_{1} or P_{2} ordinate outside the [0, 1]
range may generate bouncing effects.
When you specify an invalid cubicbezier
curve, CSS ignores the whole property.
Syntax
cubicbezier(x_{1}, y_{1}, x_{2}, y_{2})
where:
 x_{1}, y_{1}, x_{2}, y_{2}
 Are
<number>
values representing the abscissas, and ordinates of the P_{1} and P_{2} points defining the cubic Bézier curve. x_{1} and x_{2} must be in the range [0, 1] or the value is invalid.
Examples
These cubic Bézier curves are valid for use in CSS :
/* The canonical Bézier curve with four <number> in the [0,1] range. */ cubicbezier(0.1, 0.7, 1.0, 0.1) /* Using <integer> is valid as any <integer> is also a <number>. */ cubicbezier(0, 0, 1, 1) /* Negative values for ordinates are valid, leading to bouncing effects.*/ cubicbezier(0.1, 0.6, 0.2, 0) /* Values > 1.0 for ordinates are also valid. */ cubicbezier(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 w/ numerical ratios.*/ cubicbezier(0.1, red, 1.0, green) /* Abscissas must be in the [0, 1] range or the curve is not a function of time. */ cubicbezier(2.45, 0.6, 4, 0.1) /* The two points must be defined, there is no default value. */ cubicbezier(0.3, 2.1) /* Abscissas must be in the [0, 1] range or the curve is not a function of time. */ cubicbezier(1.9, 0.3, 0.2, 2.1)
The steps()
class of timingfunctions
The steps()
functional notation defines a step function dividing the domain of output values in equidistant steps.
This subclass of step functions are sometimes also called staircase functions.
steps(2, start)
steps(4, end)
Syntax
steps(number_of_steps, direction)
where:
 number_of_steps
 Is a strictly positive
<integer>
, representing the amount of equidistant treads composing the stepping function.  direction
 Is a keyword indicating if it the function is left or rightcontinuous:

start
denotes a leftcontinuous function, so that the first step happens when the animation begins;end
denotes a rightcontinuous function, so that the last step happens when the animation ends.
end
is the default.
Examples
These timing functions are valid :
/* There is 5 treads, the last one happens right before the end of the animation. */ steps(5, end) /* A twostep staircase, the first one happening at the start of the animation. */ steps(2, start) /* The second parameter is optional. */ steps(2)
These timing function 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, end) /* The amount of steps must be nonnegative. */ steps(3, start) /* There must be at least one step.*/ steps(0, end)
The frames()
class of timingfunctions
Note: The name of the frames()
timing function is currently under discussion, so it is currently disabled in browser release versions until a final decision is reached.
The frames()
functional notation defines a frames function dividing the domain of output values into equidistant intervals. The difference between frames()
and steps()
is that with frames()
, the start (0%) and end (100%) states are shown for an equal amount of time to the other intervals.
frames(2), frames(4)
Syntax
frames(number_of_frames)
where:
 number_of_frames
 Is a strictly positive
<integer>
, representing the amount of equidistant intervals composing the stepping function.
Examples
These timing functions are valid :
/* The parameter is a positive integer. */ frames(10)
Note: You can see a working transition example using the frames() function in our GitHub.
These timing function are invalid :
/* The parameter must be an <integer> and cannot be a real value, even if it is equal to one. */ frames(2.0) /* The amount of frames must be nonnegative. */ frames(3) /* There must be at least two frames.*/ frames(0)
Keywords for common timingfunctions
linear
This keyword represents the timing function cubicbezier(
0.0, 0.0, 1.0, 1.0
)
. Using this timing function, the animation goes from its initial state to its final one regularly, with a constant velocity.
ease
This keyword represents the timing function cubicbezier(0.25, 0.1, 0.25, 1.0)
. This function is similar to easeinout
, though it accelerates more sharply at the beginning, starts to slow down before the time is halfway through, and gently comes to rest.
easein
This keyword represents the timing function cubicbezier(0.42, 0.0, 1.0, 1.0)
. The animation begins slowly, then progressively accelerates until the final state is reached and the animation stops abruptly.
easeinout
This keyword represents the timing function cubicbezier(
0.42, 0.0, 0.58, 1.0
)
. With this timing function, the animation starts slowly, accelerates faster, and then slows down when approaching its final state. At the beginning, it behaves similarly to the easein
function; at the end, it is similar to the easeout
function.
easeout
This keyword represents the timing function cubicbezier(
0.0, 0.0, 0.58, 1.0
)
. The animation starts quickly and then slows down when approaching to its final state.
stepstart
This keyword represents the timing function steps(1, start)
. Using this timing function, the animation jumps immediately to the end state and stays in that position until the end of the animation.
stepend
This keyword represents the timing function steps(1, end)
. Using this timing function, the animation stays in its initial state until the end, where it directly jumps to its final position.
Specifications
Specification  Status  Comment 

CSS Animations The definition of '<singletransitiontimingfunction>' in that specification. 
Working Draft  Defines <singletimingfunction> as synonym for <singletransitiontimingfunction> of CSS Transitions Module. 
CSS Transitions The definition of '<singletransitiontimingfunction>' in that specification. 
Working Draft  Initial definition 
Browser compatibility
Feature  Firefox (Gecko)  Chrome  Internet Explorer  Opera  Safari 

Basic support  4.0 (2.0)  4.0  10.0  10.5  3.1 
cubicbezier() with ordinate ∉ [0,1] 
4.0 (2.0)  16.0  10.0  12.1  Nightly build 
steps() 
4.0 (2.0)  8.0  10.0  12.1  5.1 
frames() 
No support^{[1]}  No support^{[1]}  No support  No support^{[1]}  ? 
Feature  Firefox Mobile (Gecko)  Android  IE Phone  Opera Mobile  Safari Mobile 

Basic support  4.0 (2.0)  4.0  No support  10.0  2.0 
cubicbezier() with ordinate ∉ [0,1] 
4.0 (2.0)  (Yes)  No support  ?  ? 
steps() 
4.0 (2.0)  4.0  No support  ?  5.0 
frames() 
No support^{[1]}  ?  No support  No support  ? 
[1] The name of the frames()
timing function is currently under discussion, so it is currently disabled in browser release versions until a final decision is reached. It is currently turned on in Nightly/Canary only.