<pre class="metadata">

Title: CSS Transitions Module Level 1

Status: WD

Date: 2026-01-08

Prepare for TR: yes

Work Status: Refining

Shortname: css-transitions

Group: csswg

Level: 1

TR: https://www.w3.org/TR/css-transitions-1/

Previous version: https://www.w3.org/TR/2018/WD-css-transitions-1-20181011/

Previous version: https://www.w3.org/TR/2017/WD-css-transitions-1-20171130/

Previous version: https://www.w3.org/TR/2013/WD-css3-transitions-20131119/

ED: https://drafts.csswg.org/css-transitions/

Editor: L. David Baron, Google https://www.google.com/, https://dbaron.org/, w3cid 15393

Editor: Brian Birtles, Mozilla https://www.mozilla.org/, [email protected], w3cid 43194

Former Editor: Dean Jackson, Apple Inc https://www.apple.com/, [email protected], w3cid 42080

Former Editor: David Hyatt, Apple Inc https://www.apple.com/, [email protected], w3cid 34140

Former Editor: Chris Marrin, Apple Inc https://www.apple.com/, [email protected]

Issue Tracking: Bugzilla bugs for all levels https://www.w3.org/Bugs/Public/buglist.cgi?product=CSS&amp;component=Transitions&amp;resolution=---

Test Suite: https://wpt.fyi/results/css/css-transitions

Abstract: CSS Transitions allows property changes in CSS values to occur smoothly over a specified duration.

Status Text: <strong>This document</strong> is expected to be relatively close to last call. While some issues raised have yet to be addressed, new features are extremely unlikely to be considered for this level.

Ignored Vars: x1, x2, y1, y2

Link Defaults: css-transforms (property) transform

WPT Path Prefix: css/css-transitions/

WPT Display: open

</pre>

<style type="text/css">

div.prod { margin: 1em 2em; }

table.event-handlers {

border-collapse: collapse;

}

table.event-handlers th,

table.event-handlers td {

padding: 0.2em 1em;

}

table.event-handlers td {

border: 1px solid black;

}

</style>

<pre class="link-defaults">

spec:css2; type:property;

text:top

text:right

text:bottom

text:left

text:margin-top

text:margin-right

text:margin-bottom

text:margin-left

text:padding-top

text:padding-right

text:padding-bottom

text:padding-left

text:border-top-color

text:border-right-color

text:border-bottom-color

text:border-left-color

text:border-top-width

text:border-right-width

text:border-bottom-width

text:border-left-width

text:background-color

text:background-position

text:border-spacing

text:width

text:height

text:min-width

text:min-height

text:max-width

text:max-height

text:clip

text:letter-spacing

text:line-height

text:outline-color

text:outline-width

text:text-indent

text:font-size

text:font-weight

text:vertical-align

text:visibility

text:word-spacing

text:z-index

spec:css-backgrounds-3; type:property;

text:background-image

text:background-origin

text:box-shadow

spec:css-color-4;

type:property;

text:color

text:opacity

type:value

text:green

text:blue

text:transparent

spec:css-values-3; type:type; text:<time>

</pre>

<!-- FIXME: These overrides aren't great for dev/TR switching -->

<pre class="anchors">

url: https://www.w3.org/TR/css3-background/#shadow-inset; type: value; for: shadow; text: inset;

url: https://www.w3.org/TR/css3-background/#box-shadow-none; type: value; for: shadow; text: none;

url: https://www.w3.org/TR/CSS21/visufx.html#propdef-visibility; type: value; for: visibility; text: visible;

urlPrefix: https://www.w3.org/TR/css3-color/; type: value;

text: transparent

text: blue

text: green

url: https://dom.spec.whatwg.org/#constructing-events; type: dfn; text: event constructor;

urlPrefix: https://html.spec.whatwg.org/multipage/webappapis.html; type: dfn; spec: html

text: event handlers

text: event handler event type

text: event handler content attributes

text: event handler IDL attributes

urlPrefix: https://html.spec.whatwg.org/multipage/infrastructure.html; type: dfn; spec: html

text: HTML elements

text: dispatch; url: concept-event-dispatch

url: https://html.spec.whatwg.org/#document; type: interface; text: Document; spec: html

</pre>

Introduction {#introduction}

============================

<p><em>This section is not normative.</em>

<p>

This document introduces new CSS features to enable <em>implicit transitions</em>, which describe how CSS properties can be made to change smoothly from one value to another over a given duration.

</p>

<wpt title="crashes">

crashtests/delete-image-set.html

crashtests/size-container-transition-crash.html

crashtests/transition-during-style-attr-mutation.html

crashtests/transition-large-word-spacing-001.html

</wpt>

Value Definitions {#values}

-----------------

This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]

using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].

Value types not defined in this specification are defined in CSS Values &amp; Units [[!CSS-VALUES-3]].

Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions,

all properties defined in this specification

also accept the <a>CSS-wide keywords</a> as their property value.

For readability they have not been repeated explicitly.

<h2 id="transitions" oldids="transitions-" dfn export>

Transitions</h2>

<p>

Normally when the value of a CSS property changes, the rendered result is instantly updated, with the affected elements immediately changing from the old property value to the new property value. This section describes a way to specify gradual transitions using new CSS properties. These properties are used to animate smoothly from the old state to the new state over time.

</p>

<p>

For example, suppose that transitions of one second have been defined on the 'left' and

'background-color' properties. The following diagram illustrates the effect of updating those properties on an element, in this case moving it to the right and changing the background from red to blue. This assumes other transition parameters still have their default values.

</p>

<figure>

<img src="images/transition-example.svg" width="518"

alt="Example showing the initial, intermediate, and final states of a box whose color and position is interpolated">

<figcaption>

Transitions of 'left' and 'background-color'.

</figcaption>

</figure>

<p>

Transitions are a presentational effect. The <a>computed value</a> of a property transitions over time from the old value to the new value. Therefore if a script queries the <a>computed value</a> of a property (or other data depending on it) as it is transitioning, it will see an intermediate value that represents the current animated value of the property.

</p>

<p>

The transition for a property is defined using a number of new properties. For example:

</p>

<div class="example">

<p style="display:none">

Example(s):

</p>

<pre>

div {

transition-property: opacity;

transition-duration: 2s;

}

</pre>The above example defines a transition on the 'opacity' property that, when a new value is assigned to it, will cause a smooth change between the old value and the new value over a period of two seconds.

</div>

<p>

Each of the transition properties accepts a comma-separated list, allowing multiple transitions to be defined, each acting on a different property. In this case, the individual transitions take their parameters from the same index in all the lists. For example:

</p>

<div class="example">

<p style="display:none">

Example(s):

</p>

<pre>

div {

transition-property: opacity, left;

transition-duration: 2s, 4s;

}

</pre>This will cause the 'opacity' property to transition over a period of two seconds and the left property to transition over a period of four seconds.

</div>

<p id="list-matching">

In the case where the lists of values in transition properties

do not have the same length, the length of the

'transition-property' list determines the number of items in

each list examined when starting transitions. The lists are

matched up from the first value: excess values at the end are

not used. If one of the other properties doesn't have enough

comma-separated values to match the number of values of

'transition-property', the user agent must calculate its used value by

repeating the list of values until there are enough. This

truncation or repetition does not affect the computed value.

<span class="note">

Note: This is analogous to the behavior of the 'background-*'

properties, with 'background-image' analogous to

'transition-property'.

</span>

</p>

<div class="example">

<p style="display:none">

Example(s):

</p>

<pre>

div {

transition-property: opacity, left, top, width;

transition-duration: 2s, 1s;

}

</pre>The above example defines a transition on the 'opacity' property of 2 seconds duration, a

transition on the 'left' property of 1

second duration, a transition on the 'top' property of 2 seconds duration and a

transition on the 'width' property of 1

second duration.

</div>

<p>

While authors can use transitions to create dynamically changing content,

dynamically changing content can lead to seizures in some users.

For information on how to avoid content that can lead to seizures, see

<a href="https://www.w3.org/TR/WCAG20/#seizure">Guideline 2.3:

Seizures:

Do not design content in a way that is known to cause seizures</a>

([[WCAG20]]).

</p>

<wpt>

animations/animate-with-color-mix.html

animations/color-transition-premultiplied.html

animations/text-shadow-composition.html

animations/text-shadow-interpolation.html

animations/vertical-align-composition.html

animations/vertical-align-interpolation.html

animations/z-index-interpolation.html

idlharness.html

inherit-background-color-transition.html

inheritance.html

properties-value-001.html

properties-value-002.html

properties-value-003.html

properties-value-implicit-001.html

retargetted-transition-with-box-sizing.html

shadow-root-insertion.html

transition-base-response-001.html

transition-base-response-002.html

transition-base-response-003.html

transition-in-iframe-001.html

</wpt>

<span id="the-transition-property-property-">The 'transition-property' Property</span> {#transition-property-property}

----------------------------------------------------------------------------------------------------------------------

<p>

The 'transition-property' property specifies the name of the CSS property to which the transition is applied.

</p>

<pre class="propdef">

Name: transition-property

Value: none | <<single-transition-property>>#

Initial: all

Applies to: all elements

Inherited: no

Percentages: N/A

Computed value: the keyword ''transition-property/none'' else a list of identifiers

Animation type: not animatable

</pre>

<wpt>

animations/move-after-transition.html

animations/transition-end-event-shorthands.html

parsing/transition-property-computed.html

parsing/transition-property-invalid.html

parsing/transition-property-valid.html

pseudo-element-transform.html

pseudo-elements-001.html

pseudo-elements-002.html

transition-background-position-with-edge-offset.html

transition-property-001.html

transition-property-002.html

transition-property-003-manual.html

transition-property-004-manual.html

transition-property-005-manual.html

transition-property-006-manual.html

transition-property-007-manual.html

transition-property-008-manual.html

transition-property-009-manual.html

transition-property-010-manual.html

transition-property-011-manual.html

transition-property-012-manual.html

transition-property-013-manual.html

transition-property-014-manual.html

transition-property-015-manual.html

transition-property-016-manual.html

transition-property-017-manual.html

transition-property-018-manual.html

transition-property-019-manual.html

transition-property-020-manual.html

transition-property-021-manual.html

transition-property-022-manual.html

transition-property-023-manual.html

transition-property-024-manual.html

transition-property-025-manual.html

transition-property-026-manual.html

transition-property-027-manual.html

transition-property-028-manual.html

transition-property-029-manual.html

transition-property-030-manual.html

transition-property-031-manual.html

transition-property-032-manual.html

transition-property-033-manual.html

transition-property-034-manual.html

transition-property-035-manual.html

transition-property-036-manual.html

transition-property-037-manual.html

transition-property-038-manual.html

transition-property-039-manual.html

transition-property-040-manual.html

transition-property-041-manual.html

transition-property-042-manual.html

transition-property-043-manual.html

transition-property-044-manual.html

transition-property-045-manual.html

transition-test.html

transition-zero-duration-with-delay.html

</wpt>

<div class="prod">

<dfn type id="single-transition-property">&lt;single-transition-property&gt;</dfn> = ''transition-property/all'' | <<custom-ident>>

</div>

<p>

A value of

<dfn value for="transition-property">none</dfn>

means that no property will transition.

Otherwise, a list of properties to be transitioned, or the

keyword <dfn value for="transition-property">all</dfn>

which indicates that all properties are to be

transitioned, is given.

</p>

<p>

If one of the identifiers listed is not a recognized property

name, the implementation must

still start transitions on the animatable properties in the

list using the duration, delay, and timing function at their

respective indices in the lists for 'transition-duration',

'transition-delay', and 'transition-timing-function'. In other

words, unrecognized properties must be kept in

the list to preserve the matching of indices.

</p>

<p>

The <<custom-ident>> production in <<single-transition-property>>

also excludes the keyword ''transition-property/none'',

in addition to the keywords always excluded from <<custom-ident>>.

This means that

''transition-property/none'', ''inherit'', and ''initial'' are not

permitted as items within a list of more that one identifier;

any list that uses them is syntactically invalid.

</p>

<p>

For the keyword ''transition-property/all'',

or if one of the identifiers listed is a

shorthand property, implementations must start transitions for

all its longhand sub-properties (or, for

''transition-property/all'', all properties), using the duration, delay,

and timing function at the index corresponding to the shorthand.

</p>

<p>

If a property is specified multiple times in the value of

'transition-property' (either on its own, via a shorthand that

contains it, or via the ''transition-property/all'' value), then the transition that

starts uses the duration, delay, and timing function at the

index corresponding to the <em>last</em> item in the value of

'transition-property' that calls for animating that property.

</p>

<p class="note">

Note: The ''transition-property/all'' value and 'all' shorthand

property work in similar ways, so the

''transition-property/all'' value is just like a shorthand that

covers all properties.

</p>

<span id="the-transition-duration-property-">The 'transition-duration' Property</span> {#transition-duration-property}

----------------------------------------------------------------------------------------------------------------------

<p>

The 'transition-duration' property defines the length of time that a transition takes.

</p>

<pre class="propdef">

Name: transition-duration

Value: <<time [0s,∞]>>#

Initial: ''0s''

Applies to: all elements

Inherited: no

Percentages: N/A

Computed value: list, each item a duration

Animation type: not animatable

</pre>

<wpt>

infinite-duration-crash.html

parsing/transition-duration-computed.html

parsing/transition-duration-invalid.html

parsing/transition-duration-valid.html

transition-duration-001.html

transition-duration-002-manual.html

transition-duration-003-manual.html

transition-duration-004-manual.html

transition-duration-shorthand.html

</wpt>

<p>

This property specifies how long the transition from the old value to the new value should take. By default the value is ''0s'', meaning that the transition is immediate (i.e. there will be no animation). A negative value for 'transition-duration' renders the declaration invalid.

</p>

<span id="transition-timing-function_tag">The 'transition-timing-function' Property</span> {#transition-timing-function-property}

---------------------------------------------------------------------------------------------------------------------------------

<p>

The 'transition-timing-function' property

describes how the intermediate values used during a transition will be

calculated. It allows for a transition to change speed over its

duration. These effects are commonly called <em>easing</em> functions.

</p>

<p>

Timing functions are defined in the separate CSS Easing Functions module

[[!css-easing-1]].

The <a spec=css-easing>input progress value</a> used is the percentage

of the transition duration, and the <a spec=css-easing>output progress

value</a> is used as the <var>p</var> value when [=interpolating=]

the property value (see [[#application]]).

</p>

<pre class="propdef">

Name: transition-timing-function

Value: <<easing-function>>#

Initial: ''transition-timing-function/ease''

Applies to: all elements

Inherited: no

Percentages: N/A

Computed value: as specified

Animation type: not animatable

</pre>

<wpt>

animations/transition-timing-function.html

parsing/transition-timing-function-computed.html

parsing/transition-timing-function-invalid.html

parsing/transition-timing-function-valid.html

transition-timing-function-002-manual.html

transition-timing-function-003-manual.html

transition-timing-function-004-manual.html

transition-timing-function-005-manual.html

transition-timing-function-006-manual.html

transition-timing-function-010-manual.html

</wpt>

<span id="the-transition-delay-property-">The 'transition-delay' Property</span> {#transition-delay-property}

-------------------------------------------------------------------------------------------------------------

<p>

The 'transition-delay' property defines when the transition will start. It allows a transition to begin execution after some period of time from when it is applied. A 'transition-delay' value of ''0s'' means the transition will execute as soon as the property is changed. Otherwise, the value specifies an offset from the moment the property is changed, and the transition will delay execution by that offset.

</p>

<p>

If the value for 'transition-delay' is a negative time offset then the transition will execute the moment the property is changed, but will appear to have begun execution at the specified offset. That is, the transition will appear to begin part-way through its play cycle. In the case where a transition has implied starting values and a negative 'transition-delay', the starting values are taken from the moment the property is changed.

</p>

<pre class="propdef">

Name: transition-delay

Value: <<time>>#

Initial: ''0s''

Applies to: all elements

Inherited: no

Percentages: N/A

Computed value: list, each item a duration

Animation type: not animatable

</pre>

<wpt>

parsing/transition-delay-computed.html

parsing/transition-delay-invalid.html

parsing/transition-delay-valid.html

transition-delay-000-manual.html

transition-delay-001.html

transition-delay-002-manual.html

transition-delay-003-manual.html

</wpt>

<span id="the-transition-shorthand-property-">The 'transition' Shorthand Property</span> {#transition-shorthand-property}

-------------------------------------------------------------------------------------------------------------------------

<p>

The 'transition' shorthand property combines the four properties described above into a single property.

</p>

<pre class="propdef shorthand">

Name: transition

Value: <<single-transition>>#

Applies to: all elements

Inherited: no

Percentages: N/A

Animation type: not animatable

</pre>

<wpt>

parsing/transition-computed.html

parsing/transition-invalid.html

parsing/transition-valid.html

parsing/transition-shorthand.html

transition-001.html

</wpt>

<div class="prod">

<dfn type id="single-transition">&lt;single-transition&gt;</dfn> = [ ''transition-property/none'' | <<single-transition-property>> ] || <<time>> || <<easing-function>> || <<time>>

</div>

<p>

Note that order is important within the items in this property:

the first value that can be parsed as a time is assigned to the

transition-duration,

and the second value that can be parsed as a time is assigned to

transition-delay.

</p>

<p>

If there is more than one <<single-transition>> in the shorthand,

and any of the transitions has

''transition-property/none'' as the <<single-transition-property>>,

then the declaration is invalid.

</p>

Starting of transitions {#starting}

===================================

<p>

Implementations must maintain a set of

<dfn export lt="running transition">running transitions</dfn>,

each of which applies to a specific element and non-shorthand

property. Each of these transitions also has a

<dfn export for="transition">start time</dfn>, <dfn export for="transition">end time</dfn>,

<dfn export for="transition">start value</dfn>, <dfn export for="transition">end value</dfn>,

<dfn export for="transition">reversing-adjusted start value</dfn>, and <dfn export for="transition">reversing shortening factor</dfn>.

Transitions are added to this set as described in this section,

and are removed from this set

when they [=transition/complete=]

or when implementations are required to <dfn export for="transition">cancel</dfn> them.

<span class="note">

For the rationale behind the <a>reversing-adjusted start value</a>

and <a>reversing shortening factor</a>, see [[#reversing]].

</span>

</p>

<wpt>

transitioncancel-003.html

</wpt>

<p>

Implementations must also maintain a set of

<dfn export lt="completed transition">completed transitions</dfn>,

each of which

(like <a>running transitions</a>)

applies to a specific element and non-shorthand property.

<span class="note">

This specification maintains the invariant that

there is never both a <a>running transition</a> and

a <a>completed transition</a> for the same property and element.

</span>

</p>

<p>

If an element is no longer in the document,

implementations must [=transition/cancel=] any <a>running transitions</a>

on it and remove transitions on it from the

<a>completed transitions</a>.

</p>

<div class="note">

<p>

This set of completed transitions

needs to be maintained

in order to prevent

transitions from repeating themselves in certain cases,

i.e., to maintain the invariant

that this specification tries to maintain

that unrelated style changes do not trigger transitions.

</p>

<p class="example">

An example where maintaining the set of completed transitions

is necessary would be a transition on

an inherited property,

where the parent specifies a transition of that property for

a longer duration (say, ''transition: 4s text-indent'')

and a child element that inherits the parent's value specifies

a transition of the same property for a shorter duration

(say, ''transition: 1s text-indent'').

Without the maintenance of this set of completed transitions,

implementations could start additional transitions on the child

after the initial 1 second transition on the child completes.

</p>

</div>

<p>

Various things can cause the <a>computed values</a> of properties

on an element to change.

These include

insertion and removal of elements from the document tree

(which both changes whether those elements have <a>computed values</a> and

can change the styles of other elements through selector matching),

changes to the document tree that cause

changes to which selectors match elements,

changes to style sheets or style attributes,

and other things.

This specification does not define when <a>computed values</a> are updated,

beyond saying that implementations must not

use, present, or display something resulting from the CSS

cascading, value computation, and inheritance process [[!CSS3CASCADE]]

without updating the <a>computed value</a>

(which means merely that implementations cannot avoid

meeting requirements of this specification

by claiming not to have updated the <a>computed value</a>

as part of handling a style change).

However,

when an implementation updates the <a>computed value</a> of a

property on an element

to reflect one of these changes,

or computes the <a>computed value</a> of a property on an element

newly added to the document,

it must update the <a>computed value</a>

for all properties and elements to reflect all

of these changes at the same time

(or at least it must be undetectable that it was done at a

different time).

This processing of a set of simultaneous style changes is called a

<dfn export>style change event</dfn>.

(Implementations typically have a <a>style change event</a> to

correspond with their desired screen refresh rate,

and when up-to-date computed style or layout information is needed

for a script API that depends on it.)

</p>

<wpt>

render-blocking/no-transition-from-ua-to-blocking-stylesheet.html

</wpt>

<p>

Since this specification does not define

when a <a>style change event</a> occurs,

and thus what changes to computed values are considered simultaneous,

authors should be aware that changing any of the transition

properties a small amount of time after making a change that

might transition can result in behavior that varies between

implementations, since the changes might be considered

simultaneous in some implementations but not others.

</p>

<p>

When a <a>style change event</a> occurs,

implementations must start transitions based on

the <a>computed values</a> that changed in that event.

If an element is not in the document during that

style change event or was not in the document during

the previous style change event,

then transitions are not started for that element

in that style change event.

Otherwise,

define the <dfn export>before-change style</dfn> as

the <a>computed values</a> of all properties on the element as of

the previous <a>style change event</a>,

except with any styles derived from declarative

animations such as CSS Transitions, CSS Animations

([[CSS3-ANIMATIONS]]),

and SMIL Animations ([[SMIL-ANIMATION]], [[SVG11]])

updated to the current time.

Likewise, define the <dfn export>after-change style</dfn> as

the <a>computed values</a> of all properties

on the element based on the information

known at the start of that <a>style change event</a>,

but using the computed values of the 'animation-*' properties from the

<a>before-change style</a>,

excluding any styles from CSS Transitions in the computation,

and inheriting from

the <a>after-change style</a> of the parent.

Note that this means the <a>after-change style</a> does not differ from

the <a>before-change style</a> due to newly created or canceled CSS

Animations.

</p>

<wpt>

after-change-style-inherited-try-fallback.html

after-change-style-inherited.html

animations/change-duration-during-transition.html

</wpt>

<div class="note">

<p>

Note that this definition of the <a>after-change style</a>

means that a single change

can start a transition on the same property

on both an ancestor element and its descendant element.

This can happen when a property change is inherited

from one element with 'transition-*' properties

that say to animate the changing property

to another element with 'transition-*' properties

that also say to animate the changing property.

</p>

<p>

When this happens, both transitions will run,

and the transition on the descendant will override

the transition on the ancestor

because of the normal

CSS cascading and inheritance rules ([[CSS3CASCADE]]).

</p>

<p>

If the transition on the descendant completes before

the transition on the ancestor,

the descendant will then resume inheriting

the (still transitioning) value from its parent.

This effect is likely not a desirable effect,

but it is essentially doing what the author asked for.

</p>

</div>

<p>

For each element with a <a>before-change style</a> and

an <a>after-change style</a>,

and each property (other than shorthands),

define the <dfn export>matching transition-property value</dfn> as

the last value in the

'transition-property' in the element's <a>after-change style</a>

that matches the property,

as described in

[[#transition-property-property]].

If there is such a value, then corresponding to it, there is

a <dfn export>matching transition duration</dfn>,

a <dfn export>matching transition delay</dfn>, and

a <dfn export>matching transition timing function</dfn>

in the values in the <a>after-change style</a> of

'transition-duration', 'transition-delay', and 'transition-timing-function'

(see <a href="#list-matching">the rules on matching lists</a>).

Define the <dfn export for="transition">combined duration</dfn> of the transition

as the sum of max(<a>matching transition duration</a>, ''0s'') and

the <a>matching transition delay</a>.

</p>

When comparing the [=before-change style=] and [=after-change style=]

for a given property,

the property values are <dfn>transitionable</dfn>

if they have an [=animation type=] that is

<em>neither</em> [=not animatable=] <em>nor</em> [=discrete=].

Note: Even if a <em>property</em> is defined to have an [=animation type=]

that is <em>not</em> [=discrete=],

for a particular pair of <em>property values</em>

the [=animation type=] may be [=discrete=].

For example,

the [=animation type=] of the 'box-shadow' property is

<a lt="combining shadow lists">shadow list</a>,

which defines that

when the ''shadow/inset'' keyword is absent in one value

but present in the other,

[=discrete=] animation is used.

As a result

''0px 0px black'' and ''inset 10px 10px black''

are <em>not</em> [=transitionable=].

For each element and property, the implementation must act

as follows:

<ol>

<li>

If all of the following are true:

<ul>

<li>

the element does not have

a <a>running transition</a> for the property,

</li>

<li>

the <a>before-change style</a> is different from

the <a>after-change style</a> for that property,

and the values for the property are [=transitionable=],

</li>

<li>

the element does not have a <a>completed transition</a>

for the property

or the <a>end value</a> of the <a>completed transition</a>

is different from the <a>after-change style</a> for the property,

</li>

<li>

there is a <a>matching transition-property value</a>, and

</li>

<li>

the <a>combined duration</a> is greater than ''0s'',

</li>

</ul>

then implementations must

remove the <a>completed transition</a> (if present) from the set

of completed transitions and

start a transition whose:

<ul>

<li>

[=transition/start time=] is

the time of the <a>style change event</a> plus

the <a>matching transition delay</a>,

</li>

<li>

[=transition/end time=] is

the [=transition/start time=] plus

the <a>matching transition duration</a>,

</li>

<li>

<a>start value</a> is

the value of the transitioning property

in the <a>before-change style</a>,

</li>

<li>

<a>end value</a> is

the value of the transitioning property

in the <a>after-change style</a>,

</li>

<li>

<a>reversing-adjusted start value</a> is the same as

the <a>start value</a>, and

<li>

<a>reversing shortening factor</a> is 1.

</li>

</ul>

</li>

<li>

Otherwise,

if the element has a <a>completed transition</a> for the property

and the <a>end value</a> of the <a>completed transition</a>

is different from the <a>after-change style</a> for the property,

then implementations must

remove the <a>completed transition</a> from the set of

<a>completed transitions</a>.

</li>

<li>

If the element has a <a>running transition</a> or

<a>completed transition</a> for the property,

and there is <strong>not</strong>

a <a>matching transition-property value</a>,

then implementations must

[=transition/cancel=] the <a>running transition</a>

or remove the <a>completed transition</a> from the set of

<a>completed transitions</a>.

</li>

<li>

If the element has a <a>running transition</a> for the property,

there is a <a>matching transition-property value</a>,

and the <a>end value</a> of the <a>running transition</a> is

<strong>not</strong> equal to the value of the property in the

<a>after-change style</a>, then:

<ol>

<li>

If the <a>current value</a> of the property

in the <a>running transition</a>

is equal to

the value of the property in the <a>after-change style</a>,

or if these two values are not [=transitionable=],

then implementations must

[=transition/cancel=] the <a>running transition</a>.

</li>

<li>

Otherwise, if the <a>combined duration</a> is

less than or equal to ''0s'',

or if the

<a>current value</a> of the property in the <a>running transition</a>

is not [=transitionable=] with

the value of the property in the <a>after-change style</a>,

then implementations must

[=transition/cancel=] the <a>running transition</a>.

</li>

<li>

Otherwise, if the <a>reversing-adjusted start value</a>

of the <a>running transition</a> is the same as the value of

the property in the <a>after-change style</a>

<span class="note">(see the

<a href="#reversing">section on reversing of

transitions</a> for why these case exists)</span>,

implementations must

[=transition/cancel=] the <a>running transition</a> and

start a new transition whose:

<ul>

<li>

<a>reversing-adjusted start value</a> is

the <a>end value</a> of the

<a>running transition</a>

<span class="note">(Note: This represents the logical start state of

the transition, and allows some calculations to ignore that

the transition started before that state was reached, which

in turn allows repeated reversals of the same transition to

work correctly),</span>

<li>

<a>reversing shortening factor</a>

is the absolute value, clamped to the range [0, 1],

of the sum of:

<ol>

<li>the output of the timing function of the old transition

at the time of the <a>style change event</a>,

times the <a>reversing shortening factor</a> of the

old transition</li>

<li>1 minus the <a>reversing shortening factor</a> of

the old transition.</li>

</ol>

<span class="note">Note: This represents the portion of the

space between the <a>reversing-adjusted start value</a>

and the <a>end value</a> that the old transition has

traversed (in amounts of the value, not time), except with the

absolute value and clamping to handle timing functions that

have y1 or y2 outside the range [0, 1].</span>

</li>

<li>

[=transition/start time=] is

the time of the <a>style change event</a> plus:

<ol>

<li>if the <a>matching transition delay</a>

is nonnegative,

the <a>matching transition delay</a>, or

<li>if the <a>matching transition delay</a>

is negative,

the product of

the new transition's

<a>reversing shortening factor</a> and

the <a>matching transition delay</a>,

</ol>

</li>

<li>

[=transition/end time=] is

the [=transition/start time=] plus the product of

the <a>matching transition duration</a> and

the new transition's <a>reversing shortening factor</a>,

</li>

<li>

<a>start value</a> is

the <a>current value</a> of the property

in the <a>running transition</a>,

</li>

<li>

<a>end value</a> is

the value of the property

in the <a>after-change style</a>,

</li>

</ul>

</li>

<li>

Otherwise, implementations must

[=transition/cancel=] the <a>running transition</a>

and start a new transition whose:

<ul>

<li>

[=transition/start time=] is

the time of the <a>style change event</a> plus

the <a>matching transition delay</a>,

</li>

<li>

[=transition/end time=] is

the [=transition/start time=] plus

the <a>matching transition duration</a>,

</li>

<li>

<a>start value</a> is

the <a>current value</a> of the property

in the <a>running transition</a>,

</li>

<li>

<a>end value</a> is

the value of the property

in the <a>after-change style</a>,

</li>

<li>

<a>reversing-adjusted start value</a> is the same as

the <a>start value</a>, and

<li>

<a>reversing shortening factor</a> is 1.

</li>

</ul>

</li>

</ol>

</li>