This section is not normative
CSS Transitions [CSS3-TRANSITIONS] provide a way to interpolate CSS property values when they change as a result of underlying property changes. This provides an easy way to do simple animation, but the start and end states of the animation are controlled by the existing property values, and transitions provide little control to the author on how the animation progresses.
This proposal introduces defined animations, in which the author can specify the changes in CSS properties over time as a set of keyframes. Animations are similar to transitions in that they change the presentational value of CSS properties over time. The principal difference is that while transitions trigger implicitly when property values change, animations are explicitly executed when the animation properties are applied. Because of this, animations require explicit values for the properties being animated. These values are specified using animation keyframes, described below.
Many aspects of the animation can be controlled, including how many times the animation iterates, whether or not it alternates between the begin and end values, and whether or not the animation should be running or paused. An animation can also delay its start time.
1.1. Value DefinitionsThis specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & 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 CSS-wide keywords as their property value. For readability they have not been repeated explicitly.
2. CSS Animations ModelCSS Animations affect computed property values. This effect happens by adding a specified value to the CSS cascade ([CSS3CASCADE]) (at the level for CSS Animations) that will produce the correct computed value for the current state of the animation. As defined in [CSS3CASCADE], animations override all normal rules, but are overridden by !important rules.
If at some point in time there are multiple animations specifying behavior for the same property, the animation which occurs last in the value of animation-name will override the other animations at that point.
An animation does not affect the computed value before the application of the animation (that is, when the animation-name property is set on an element) or after it is removed. Furthermore, typically an animation does not affect the computed value before the animation delay has expired or after the end of the animation, but may do so depending on the animation-fill-mode property.
While running, the animation computes the value of those properties it animates. Other values may take precedence over the animated value according to the CSS cascade ([CSS3CASCADE]).
While an animation is applied but has not finished, or has finished but has an animation-fill-mode of forwards or both, the user agent must act as if the will-change property ([css-will-change-1]) on the element additionally includes all the properties animated by the animation.
The start time of an animation is the time at which the style applying the animation and the corresponding @keyframes rule are both resolved. If an animation is specified for an element but the corresponding @keyframes rule does not yet exist, the animation cannot start; the animation will start from the beginning as soon as a matching @keyframes rule can be resolved. An animation specified by dynamically modifying the element’s style will start when this style is resolved; that may be immediately in the case of a pseudo style rule such as hover, or may be when the scripting engine returns control to the browser (in the case of style applied by script). Note that dynamically updating keyframe style rules does not start or re-start an animation.
An animation applies to an element if its name appears as one of the identifiers in the computed value of the animation-name property and the animation uses a valid @keyframes rule. Once an animation has started it continues until it ends or the animation-name is removed. Changes to the values of animation properties while the animation is running apply as if the animation had those values from when it began. For example, shortening the animation-delay may cause the animation to jump forwards or even finish immediately and dispatch an animationend event. Conversely, extending the animation-delay may cause an animation to re-start and dispatch an animationstart event.
The same @keyframes rule name may be repeated within an animation-name. Changes to the animation-name update existing animations by iterating over the new list of animations from last to first, and, for each animation, finding the last matching animation in the list of existing animations. If a match is found, the existing animation is updated using the animation properties corresponding to its position in the new list of animations, whilst maintaining its current playback time as described above. The matching animation is removed from the existing list of animations such that it will not match twice. If a match is not found, a new animation is created. As a result, updating animation-name from ‘a’ to ‘a, a’ will cause the existing animation for ‘a’ to become the second animation in the list and a new animation will be created for the first item in the list.
div {
animation-name: diagonal-slide;
animation-duration: 5s;
animation-iteration-count: 10;
}
@keyframes diagonal-slide {
from {
left: 0;
top: 0;
}
to {
left: 100px;
top: 100px;
}
}
This will produce an animation that moves an element from (0, 0) to (100px, 100px) over five seconds and repeats itself nine times (for a total of ten iterations).
Setting the display property to none will terminate any running animation applied to the element and its descendants. If an element has a display of none, updating display to a value other than none will start all animations applied to the element by the animation-name property, as well as all animations applied to descendants with display other than none.
While authors can use animations 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 Guideline 2.3: Seizures: Do not design content in a way that is known to cause seizures ([WCAG20]).
Implementations may ignore animations when the rendering medium is not interactive e.g. when printed. A future version of this specification may define how to render animations for these media.
3. Declaring KeyframesKeyframes are used to specify the values for the animating properties at various points during the animation. The keyframes specify the behavior of one cycle of the animation; the animation may iterate zero or more times.
Keyframes are specified using the @keyframes at-rule, defined as follows:
@keyframes = @keyframes <keyframes-name> { <qualified-rule-list> }
<keyframes-name> = <custom-ident> | <string>
<keyframe-block> = <keyframe-selector># { <declaration-list> }
<keyframe-selector> = from | to | <percentage [0,100]>
The <rule-list> inside of @keyframes can only contain <keyframe-block> rules.
The <declaration-list> inside of <keyframe-block> accepts any CSS property except those defined in this specification, but does accept the animation-timing-function property and interprets it specially. None of the properties interact with the cascade (so using !important on them is invalid and will cause the property to be ignored).
A @keyframes block has a name given by the <custom-ident> or <string> in its prelude. The two syntaxes are equivalent in functionality; the name is the value of the ident or string. As normal for <custom-ident>s and <string>s, the names are fully case-sensitive; two names are equal only if they are codepoint-by-codepoint equal. The <custom-ident> additionally excludes the none keyword.
For example, the following two
@keyframesrules have the same name, so the first will be ignored:
@keyframes foo { /* ... */ }
@keyframes "foo" { /* ... */ }
On the other hand, the following @keyframes rule’s name is different from the previous two rules:
@keyframes FOO { /* ... */ }
The following @keyframes rules are invalid because they use disallowed <custom-ident> values:
@keyframes initial { /* ... */ }
@keyframes None { /* ... */ }
However, those names can be specified with a <string>, so the following are both valid:
@keyframes "initial" { /* ... */ }
@keyframes "None" { /* ... */ }
The <keyframe-selector> for a <keyframe-block> consists of a comma-separated list of percentage values or the keywords from or to. The selector is used to specify the percentage along the duration of the animation that the keyframe represents. The keyframe itself is specified by the block of property values declared on the selector. The keyword from is equivalent to the value 0%. The keyword to is equivalent to the value 100%. Values less than 0% or higher than 100% are invalid and cause their <keyframe-block> to be ignored.
Note: Note that the percentage unit specifier must be used on percentage values. Therefore, 0 is an invalid keyframe selector.
If a 0% or from keyframe is not specified, then the user agent constructs a 0% keyframe using the computed values of the properties being animated. If a 100% or to keyframe is not specified, then the user agent constructs a 100% keyframe using the computed values of the properties being animated.
The <keyframe-block> contains properties and values. The properties defined by this specification are ignored in these rules, with the exception of animation-timing-function, the behavior of which is described below. In addition, properties qualified with !important are invalid and ignored.
If multiple @keyframes rules are defined with the same name, the last one in document order wins, and all preceding ones are ignored.
div {
animation-name: slide-right;
animation-duration: 2s;
}
@keyframes slide-right {
from {
margin-left: 0px;
}
50% {
margin-left: 110px;
opacity: 1;
}
50% {
opacity: 0.9;
}
to {
margin-left: 200px;
}
}
The two 50% rules from above can also be combined into an equivalent single rule as illustrated below:
@keyframes slide-right {
from {
margin-left: 0px;
}
50% {
margin-left: 110px;
opacity: 0.9;
}
to {
margin-left: 200px;
}
}
To determine the set of keyframes, all of the values in the selectors are sorted in increasing order by time. The rules within the @keyframes rule then cascade; the properties of a keyframe may thus derive from more than one @keyframes rule with the same selector value.
If a property is not specified for a keyframe, or is specified but invalid, the animation of that property proceeds as if that keyframe did not exist. Conceptually, it is as if a set of keyframes is constructed for each property that is present in any of the keyframes, and an animation is run independently for each property.
@keyframes wobble {
0% {
left: 100px;
}
40% {
left: 150px;
}
60% {
left: 75px;
}
100% {
left: 100px;
}
}
Four keyframes are specified for the animation named "wobble". In the first keyframe, shown at the beginning of the animation cycle, the value of the left property being animated is 100px. By 40% of the animation duration, left has animated to 150px. At 60% of the animation duration, left has animated back to 75px. At the end of the animation cycle, the value of left has returned to 100px. The diagram below shows the state of the animation if it were given a duration of 10s.
Animation states specified by keyframesThis specification needs to define how the value is determined from the keyframes, like the section on Application of transitions does for CSS Transitions.
3.1. Timing functions for keyframesA keyframe style rule may also declare the timing function that is to be used as the animation moves to the next keyframe.
@keyframes bounce {
from {
top: 100px;
animation-timing-function: ease-out;
}
25% {
top: 50px;
animation-timing-function: ease-in;
}
50% {
top: 100px;
animation-timing-function: ease-out;
}
75% {
top: 75px;
animation-timing-function: ease-in;
}
to {
top: 100px;
}
}
Five keyframes are specified for the animation named "bounce". Between the first and second keyframe (i.e., between 0% and 25%) an ease-out timing function is used. Between the second and third keyframe (i.e., between 25% and 50%) an ease-in timing function is used. And so on. The effect will appear as an element that moves up the page 50px, slowing down as it reaches its highest point then speeding up as it falls back to 100px. The second half of the animation behaves in a similar manner, but only moves the element 25px up the page.
A timing function specified on the to or 100% keyframe is ignored.
See the animation-timing-function property for more information.
4. Declaring AnimationsCSS Animations are defined by binding keyframes to an element using the animation-* properties. These list-valued properties, which are all longhands of the animation shorthand, form a coordinating list property group with animation-name as the coordinating list base property and each item in the coordinated value list defining the properties of a single animation effect.
Note: This is analogous to the behavior of the background-* properties, with background-image analogous to animation-name.
See CSS Values 4 § A Coordinating List-Valued Properties for how the individual animation-* property values coordinate.
4.1. The animation-name propertyThe animation-name property defines a list of animations that apply. Each name is used to select the keyframe at-rule that provides the property values for the animation. If the name does not match any keyframe at-rule, there are no properties to be animated and the animation will not execute. Furthermore, if the animation name is none then there will be no animation. This can be used to override any animations coming from the cascade. If multiple animations are attempting to modify the same property, then the animation closest to the end of the list of names wins.
The values of animation-name have the following meanings:
The animation-duration property defines duration of a single animation cycle.
If the <time> is 0s, like the initial value, the keyframes of the animation have no effect, but the animation itself still occurs instantaneously. Specifically, start and end events are fired; if animation-fill-mode is set to backwards or both, the first frame of the animation, as defined by animation-direction, will be displayed during the animation-delay. After the animation-delay the last frame of the animation, as defined by animation-direction, will be displayed if animation-fill-mode is set to forwards or both. If animation-fill-mode is set to none the animation will have no visible effect.
The animation-timing-function property describes how the animation will progress between each pair of keyframes. Timing functions are defined in the separate CSS Easing Functions module [css-easing-1].
The input progress value used is the percentage of the time elapsed between the current keyframe and the next keyframe after incorporating the effect of the animation-direction property.
During the animation-delay, the animation-timing-function is not applied.
Note: This definition is necessary because otherwise a step easing function with a step position of start would produce a backwards fill equal to the top of the first step in the function.
The output progress value is used as the p value when interpolating the property values between the current and next keyframe.
When specified in a keyframe, animation-timing-function defines the progression of the animation between the current keyframe and the next keyframe for the animating property in sorted keyframe selector order (which may be an implicit 100% keyframe).
4.4. The animation-iteration-count propertyThe animation-iteration-count property specifies the number of times an animation cycle is played. The initial value is 1, meaning the animation will play from beginning to end once. This property is often used in conjunction with an animation-direction value of alternate, which will cause the animation to play in reverse on alternate cycles.
The time window during which the animation is active (duration x iteration-count) is known as the active duration.
<single-animation-iteration-count> = infinite | <number [0,∞]>
The animation will repeat the specified number of times. If the number is not an integer, the animation will end partway through its last cycle. Negative numbers are invalid.
A value of 0 is valid and, similar to an animation-duration of 0s, causes the animation to occur instantaneously.
If the animation has a duration of 0s, it will occur instantaneously for any valid value of animation-iteration-count, including infinite.
4.5. The animation-direction propertyThe animation-direction property defines whether or not the animation should play in reverse on some or all cycles. When an animation is played in reverse the timing functions are also reversed. For example, when played in reverse an ease-in animation would appear to be an ease-out animation.
<single-animation-direction> = normal | reverse | alternate | alternate-reverse
Note: For the purpose of determining whether an iteration is even or odd, iterations start counting from 1.
4.6. The animation-play-state propertyThe animation-play-state property defines whether the animation is running or paused.
<single-animation-play-state> = running | paused
If the property is set to paused during the delay phase of the animation, the delay clock is also paused and resumes as soon as animation-play-state is set back to running.
The animation-delay property defines when the animation will start. It allows an animation to begin execution some time after it is applied, or to appear to have begun execution some time before it is applied.
A negative delay is valid. Similar to a delay of 0s, it means that the animation executes immediately, but is automatically progressed by the absolute value of the delay, as if the animation had started the specified time in the past, and so it appears to start partway through its active duration. If an animation’s keyframes have an implied starting value, the values are taken from the time the animation starts, not some time in the past.
The animation-fill-mode property defines what values are applied by the animation outside the time it is executing. By default, an animation will not affect property values between the time it is applied (the animation-name property is set on an element) and the time it begins execution (which is determined by the animation-delay property). Also, by default an animation does not affect property values after the animation ends (determined by the animation-duration and animation-iteration-count properties). The animation-fill-mode property can override this behavior. Dynamic updates to the property will be reflected by property values as needed, whether during the animation delay or after the animation ends.
<single-animation-fill-mode> = none | forwards | backwards | both
The animation shorthand property is a comma-separated list of animation definitions. Each item in the list gives one item of the value for all of the subproperties of the shorthand, which are known as the animation properties. (See the definition of animation-name for what happens when these properties have lists of different lengths, a problem that cannot occur when they are defined using only the animation shorthand.)
<single-animation> = <time [0s,∞]> || <easing-function> || <time> || <single-animation-iteration-count> || <single-animation-direction> || <single-animation-fill-mode> || <single-animation-play-state> || [ none | <keyframes-name> ]
Order is important within each animation definition: the first value in each <single-animation> that can be parsed as a <time> is assigned to the animation-duration, and the second value in each <single-animation> that can be parsed as a <time> is assigned to animation-delay.
Order is also important within each animation definition for distinguishing <keyframes-name> values from other keywords. When parsing, keywords that are valid for properties other than animation-name whose values were not found earlier in the shorthand must be accepted for those properties rather than for animation-name. Furthermore, when serializing, default values of other properties must be output in at least the cases necessary to distinguish an animation-name that could be a value of another property, and may be output in additional cases.
5. Animation EventsSeveral animation-related events are available through the DOM Event system. The start and end of an animation, and the end of each iteration of an animation, all generate DOM events. An element can have multiple properties being animated simultaneously. This can occur either with a single animation-name value with keyframes containing multiple properties, or with multiple animation-name values. For the purposes of events, each animation-name specifies a single animation. Therefore an event will be generated for each animation-name value and not necessarily for each property being animated.
Any animation for which a valid keyframe rule is defined will run and generate events; this includes animations with empty keyframe rules.
The time the animation has been running is sent with each event generated. This allows the event handler to determine the current iteration of a looping animation or the current position of an alternating animation. This time does not include any time the animation was in the paused play state.
5.1. TheAnimationEvent Interface
The AnimationEvent interface provides specific contextual information associated with Animation events.
[Exposed=Window] interface5.1.2. AttributesAnimationEvent: Event { constructor(CSSOMStringtype, optional AnimationEventInitanimationEventInitDict= {}); readonly attribute CSSOMString animationName; readonly attribute double elapsedTime; readonly attribute CSSOMString pseudoElement; }; dictionaryAnimationEventInit: EventInit { CSSOMStringanimationName= ""; doubleelapsedTime= 0.0; CSSOMStringpseudoElement= ""; };
animationName, of type CSSOMString, readonly
elapsedTime, of type double, readonly
pseudoElement, of type CSSOMString, readonly
AnimationEvent(type, animationEventInitDict) is an event constructor.
AnimationEvent
The different types of animation events that can occur are:
animationstart
animationstart event occurs at the start of the animation. If there is an animation-delay then this event will fire once the delay period has expired.
A negative delay will cause the event to fire with an elapsedTime equal to the absolute value of the delay capped to the active duration of the animation, that is, min(max(-animation-delay, 0), active duration); in this case the event will fire whether animation-play-state is set to running or paused.
animationend
animationend event occurs when the animation finishes. In this case the value of the elapsedTime member of the event is equal to the active duration.
animationiteration
animationiteration event occurs at the end of each iteration of an animation, except when an animationend event would fire at the same time. This means that this event does not occur for animations with an iteration count of one or less.
The elapsedTime member in this case is equal to the product of the current iteration and animation-duration where the current iteration is the zero-based index of the new iteration. For example, assuming no negative animation-delay, after one iteration completes the current iteration would be one.
animationcancel
animationcancel event occurs when the animation stops running in a way that does not fire an animationend event, such as a change in the animation-name that removes the animation, or the animating element or one of its ancestors becoming display:none.
The elapsedTime member for this event indicates the number of seconds that had elapsed since the beginning of the animation at the moment when the animation was canceled. This excludes any time where the animation was paused. If the animation had a negative animation-delay, the beginning of the animation is the moment equal to the absolute value of animation-delay seconds prior to when the animation was actually triggered. Alternatively, if the animation had a positive animation-delay and the event is fired before the animation’s delay has expired, the elapsedTime will be zero.
Document objects, and Window objects
The following are the event handlers (and their corresponding event handler event types) that must be supported by all HTML elements, as both event handler content attributes and event handler IDL attributes; and that must be supported by all Document and Window objects, as event handler IDL attributes:
CSS animations are exposed to the CSSOM through a pair of new interfaces describing the keyframes.
Note: the interfaces defined below reflect the interoperable API available as of this level of the specification. Future levels may deprecate parts of this API and extend others.
6.1. TheCSSRule Interface
The following two rule types are added to the CSSRule interface. They provide identification for the new keyframe and keyframes rules.
partial interface CSSRule {
const unsigned short KEYFRAMES_RULE = 7;
const unsigned short KEYFRAME_RULE = 8;
};
6.2. The CSSKeyframeRule Interface
The CSSKeyframeRule interface represents the style rule for a single key.
[Exposed=Window]
interface CSSKeyframeRule : CSSRule {
attribute CSSOMString keyText;
[SameObject, PutForwards=cssText] readonly attribute CSSStyleProperties style;
};
6.2.2. Attributes
keyText, of type CSSOMString
If keyText is updated with an invalid keyframe selector, a SyntaxError exception must be thrown and the value of keyText must remain unchanged.
style, of type CSSStyleProperties, readonly
CSSStyleProperties object for the keyframe rule, with the following properties:
CSSKeyframeRule).
CSSKeyframesRule Interface
The CSSKeyframesRule interface represents a complete set of keyframes for a single animation.
[Exposed=Window]
interface CSSKeyframesRule : CSSRule {
attribute CSSOMString name;
readonly attribute CSSRuleList cssRules;
readonly attribute unsigned long length;
getter CSSKeyframeRule (unsigned long index);
undefined appendRule(CSSOMString rule);
undefined deleteRule(CSSOMString select);
CSSKeyframeRule? findRule(CSSOMString select);
};
6.3.2. Attributes
name, of type CSSOMString
cssRules, of type CSSRuleList, readonly
length, of type unsigned long, readonly
The indexed property getter returns the CSSKeyframeRule from the list of keyframes at the indicated position.
Parameters:
index of type unsigned long
Return Value:
CSSKeyframeRule
undefined if there is no rule at the specific index.
No Exceptions
6.3.4. TheappendRule method
The appendRule method appends the passed CSSKeyframeRule at the end of the keyframes rule.
Parameters:
rule of type CSSOMString
No Return Value
No Exceptions
6.3.5. ThedeleteRule method
The deleteRule method deletes the last declared CSSKeyframeRule matching the specified keyframe selector. If no matching rule exists, the method does nothing.
Parameters:
select of type CSSOMString
The number and order of the values in the specified keyframe selector must match those of the targeted keyframe rule(s). The match is not sensitive to white space around the values in the list.
No Return Value
No Exceptions
6.3.6. ThefindRule method
The findRule returns the last declared CSSKeyframeRule matching the specified keyframe selector. If no matching rule exists, the method does nothing.
Parameters:
select of type CSSOMString
The number and order of the values in the specified keyframe selector must match those of the targeted keyframe rule(s). The match is not sensitive to white space around the values in the list.
Return Value:
CSSKeyframeRule
No Exceptions
For example, given the following animation:
@keyframes colorful-diagonal-slide {
from {
left: 0;
top: 0;
}
10% {
background-color: blue;
}
10% {
background-color: green;
}
25%, 75% {
background-color: red;
}
100% {
left: 100px;
top: 100px;
}
}
Assuming the variable anim holds a reference to a CSSKeyframesRule object for this animation, then:
anim.deleteRule('10%');
var tenPercent = anim.findRule('10%');
will start by deleting the last 10% rule i.e. the green background color rule; then find the remaining blue background rule and return it into tenPercent.
The following:
var red = anim.findRule('75%');
will set red to null. The full selector for the red background color rule must be used instead:
var red = anim.findRule('25%,75%');
Since from maps to 0% and to maps to 100%, we can find these rules using either value:
var from = anim.findRule('0%'); // Returns from { left: 0; top: 0; } rule
var to = anim.findRule('to'); // Returns 100% { left: 100px; top: 100px; } rule
6.4. Extensions to the GlobalEventHandlers Interface Mixin
This specification extends the GlobalEventHandlers interface mixin from HTML to add event handler IDL attributes for animation events as defined in § 5.3 Event handlers on elements, Document objects, and Window objects.
partial interface mixin GlobalEventHandlers {
attribute EventHandler onanimationstart;
attribute EventHandler onanimationiteration;
attribute EventHandler onanimationend;
attribute EventHandler onanimationcancel;
};
7. Privacy Considerations
No privacy concerns have been reported on this specification.
8. Security ConsiderationsNo security concerns have been reported on this specification.
9. Changes 9.1. Changes since the Working Draft of 11 October 2018The following substantive changes were made:
Thanks especially to the feedback from Tab Atkins, Brian Birtles, Shane Stephens, Carine Bournez, Christian Budde, Anne van Kesteren, Øyvind Stenhaug, Estelle Weyl, and all the rest of the www-style community.
11. Other open issuesNeed to specify how keyframes interact.
12. Working Group Resolutions that are pending editingThis section is informative and temporary.
The editors are currently behind on editing this spec. The following working group resolutions still need to be edited in:
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:
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.
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.
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 component 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.
Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.
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.
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