CSS Timing Functions Level 1

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This CSS module describes a way for authors to define a transformation to be applied to the time of an animation. This can be used to produce animations that mimic physical phenomena such as momentum or to cause the animation to move in discrete steps producing robot-like movement.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-timing” in the title, preferably like this: “[css-timing] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 February 2018 W3C Process Document.

1. Introduction

This section is not normative.

It is often desirable to control the rate at which an animation progresses. For example, gradually increasing the speed at which an element moves can give the element a sense of weight as it appears to gather momentum. This can be used to produce user intuitive interface elements or convincing cartoon props that behave like their physical counterparts. Alternatively, it is sometimes desirable for animation to move forwards in distinct steps such as a segmented wheel that rotates such that the segments always appear in the same position.

Timing functions provide a means to transform animation time by taking an input progress value and producing a corresponding transformed output progress value.

Example of a timing function that produces an ease-in effect.
Example of a timing function that produces an ease-in effect.
Given an input progress of 0.7, the timing function scales the value to produce an output progress of 0.52.
By applying this timing function, the animation will progress more slowly at first but then gradually progress more quickly.

2. Timing functions

A timing function takes an input progress value and produces an output progress value.

A timing function must be a pure function meaning that for a given set of inputs, it always produces the same output progress value.

The input progress value is a real number in the range [-∞, ∞]. Typically, the input progress value is in the range [0, 1] but this may not be the case when timing functions are chained together.

The output progress value is a real number in the range [-∞, ∞].

Some types of timing function also take an additional boolean before flag input which is defined subsequently.

This specification defines four types of timing functions whose definitions follow.

The syntax for specifying a timing function is as follows:

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

2.1. The linear timing function: linear

The linear timing function is an identity function meaning that its output progress value is equal to the input progress value for all inputs.

The syntax for the linear timing function is simply the linear keyword.

2.2. Cubic Bézier timing functions: ease, ease-in, ease-out, ease-in-out, cubic-bezier()

A cubic Bézier timing function is a type of timing function defined by four real numbers that specify the two control points, P1 and P2, of a cubic Bézier curve whose end points P0 and P3 are fixed at (0, 0) and (1, 1) respectively. The x coordinates of P1 and P2 are restricted to the range [0, 1].

A cubic Bezier curve used as a timing function.
A cubic Bézier curve used as a timing function.
The shape of the curve is determined by the location of the control points P1 and P2.
Input progress values serve as x values of the curve, whilst the y values are the output progress values.

A cubic Bézier timing function has the following syntax (using notation from [CSS3VAL]):

<cubic-bezier-timing-function> = ease | ease-in | ease-out | ease-in-out | cubic-bezier(<number>, <number>, <number>, <number>)

The meaning of each value is as follows:


Equivalent to cubic-bezier(0.25, 0.1, 0.25, 1).


Equivalent to cubic-bezier(0.42, 0, 1, 1).


Equivalent to cubic-bezier(0, 0, 0.58, 1).


Equivalent to cubic-bezier(0.42, 0, 0.58, 1).

cubic-bezier(<number>, <number>, <number>, <number>)

Specifies a cubic Bézier timing function. The four numbers specify points P1 and P2 of the curve as (x1, y1, x2, y2). Both x values must be in the range [0, 1] or the definition is invalid.

The keyword values listed above are illustrated below.

The timing functions produced by keyword values.
The timing functions produced by each of cubic Bézier timing function keyword values.

2.2.1. Output of a cubic bézier timing function

The mapping from input progress to output progress is performed by determining the corresponding y value (output progress value) for a given x value (input progress value). The evaluation of this curve is covered in many sources such as [FUND-COMP-GRAPHICS].

For input progress values outside the range [0, 1], the curve is extended infinitely using tangent of the curve at the closest endpoint as follows:

2.3. Step timing functions: step-start, step-end, steps()

A step timing function is a type of timing function that divides the input time into a specified number of intervals that are equal in length. It is defined by a number of steps, and a step position. It has following syntax:

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

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

The meaning of each value is as follows:


Computes to steps(1, start)


Computes to steps(1, end)

Example step timing keywords.
Example step timing function keyword values.
steps(<integer>[, <step-position> ]?)

The first parameter specifies the number of intervals in the function. 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.

The second parameter, which is optional, specifies the step position using one of the following values:


The first rise occurs at input progress value of 0.


The last rise occurs at input progress value of 1.


All rises occur within the range (0, 1).


The first rise occurs at input progress value of 0 and the last rise occurs at input progress value of 1.


Behaves as jump-start.


Behaves as jump-end.

If the second parameter is omitted, the value end is assumed.

These values are illustrated below:

Example step timing functions.
Example step timing functions.

2.3.1. Output of a step timing function

At the exact point where a step occurs, the result of the function is conceptually the top of the step. However, an additional before flag passed as input to the step timing function, if true, will cause the result of the function to correspond to the bottom of the step at the step point.

As an example of how the before flag affects the behavior of this function, consider an animation with a step timing function whose step position is start and which has a positive delay and backwards fill.

For example, using CSS animation:

animation: moveRight 5s 1s steps(5, start);

During the delay phase, the input progress value will be zero but if the before flag is set to indicate that the animation has yet to reach its animation interval, the timing function will produce zero as its output progress value, i.e. the bottom of the first step.

At the exact moment when the animation interval begins, the input progress value will still be zero, but the before flag will not be set and hence the result of the timing function will correspond to the top of the first step.

For the purposes of calculating the output progress value, the step position start is considered equivalent to jump-start. Likewise end is considered equivalent to jump-end. As a result, the following algorithm does not make explicit reference to start or end.

Note: User agents must still differentiate between jump-start and start for the purpose of serialization (see §2.4 Serialization).

The output progress value is calculated from the input progress value and before flag as follows:

  1. Calculate the current step as floor(input progress value × steps).

  2. If the step position property is one of:

    increment current step by one.

  3. If both of the following conditions are true:

    decrement current step by one.

  4. If input progress value ≥ 0 and current step < 0, let current step be zero.

  5. Calculate jumps based on the step position as follows:

    jump-start or jump-end



    steps - 1


    steps + 1

  6. If input progress value ≤ 1 and current step > jumps, let current step be jumps.

    Steps 5 and 6 in this procedure ensure that given an input progress value in the range [0, 1], a step timing function does not produce an output progress value outside that range.

    For example, although mathematically we might expect that a step timing function with a step position of jump-start would step up (i.e. beyond 1) when the input progress value is 1, intuitively, when we apply such a timing function to a forwards-filling animation, we expect it to produce an output progress value of 1 as the animation fills forwards.

    A similar situation arises for a step timing function with a step position of jump-end when applied to an animation during its delay phase.

  7. The output progress value is current step / jumps.

2.4. Serialization

Timing functions are serialized using the common serialization patterns defined in [CSSOM] with the following additional requirements:

3. Changes since last publication

The following changes have been made since the 21 February 2017 Working Draft:

4. Acknowledgements

This specification is based on the CSS Transitions specification edited by L. David Baron, Dean Jackson, David Hyatt, and Chris Marrin. The editors would also like to thank Douglas Stockwell, Steve Block, Tab Atkins, Rachel Nabors, Martin Pitt, and the Animation at Work slack community for their feedback and contributions.


Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

Partial Implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported property values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at https://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.


Terms defined by this specification

Terms defined by reference


Normative References

Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 14 August 2018. WD. URL: https://www.w3.org/TR/css-values-4/
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 14 August 2018. CR. URL: https://www.w3.org/TR/css-values-3/
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

Informative References

Simon Pieters; Glenn Adams. CSS Object Model (CSSOM). 17 March 2016. WD. URL: https://www.w3.org/TR/cssom-1/
Peter Shirley; Michael Ashikhmin; Steve Marschner. Fundamentals of Computer Graphics. 2009.