Fri, 20 Apr 2012 15:47:11 -0300
Mention that we're talking about computed values, per http://lists.w3.org/Archives/Public/www-style/2012Apr/0564.html .
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6 <title>CSS Transitions</title>
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24 <!--logo-->
26 <h1>CSS Transitions</h1>
28 <h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
29 <dl>
30 <dt>This version:
31 <dd>
32 <a href="[VERSION]">
33 http://dev.w3.org/csswg/css3-transitions/</a>
34 <!--http://www.w3.org/TR/[YEAR]/WD-[SHORTNAME]-[CDATE]/-->
35 <dt>Latest version:
36 <dd><a href="http://www.w3.org/TR/css3-transitions/">
37 [LATEST]</a>
38 <dt>Editor's draft:
39 <dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
40 <dt>Previous version:
41 <dd><a href="http://www.w3.org/TR/2012/WD-css3-transitions-20120403/">
42 http://www.w3.org/TR/2012/WD-css3-transitions-20120403/</a>
43 <dt id="editors-list">Editors:
44 <dd><a href="mailto:dino@apple.com">Dean Jackson</a> (<a
45 href="http://www.apple.com/">Apple Inc</a>)
46 <dd><a href="mailto:hyatt@apple.com">David Hyatt</a> (<a
47 href="http://www.apple.com/">Apple Inc</a>)
48 <dd><a href="mailto:cmarrin@apple.com">Chris Marrin</a> (<a
49 href="http://www.apple.com/">Apple Inc</a>)
50 <dd class=vcard><a class=fn href="http://dbaron.org/">L. David Baron</a> (<a
51 class=org href="http://www.mozilla.org/">Mozilla</a>)
53 <dt>Issues list:
54 <dd><a href="https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&product=CSS&component=Transitions&resolution=---&cmdtype=doit">in Bugzilla</a>
56 <dt>Discussion:</dt>
57 <dd><a href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a> with subject line “<kbd>[[SHORTNAME]] <var>… message topic …</var></kbd>”
59 <dt>Test suite:
60 <dd>none yet
61 </dl>
63 <!--copyright-->
65 <hr title="Separator for header">
66 </div>
68 <h2 class="no-num no-toc" id="abstract">Abstract</h2>
70 <p>CSS Transitions allows property changes in CSS values to occur smoothly
71 over a specified duration.
73 <h2 class="no-num no-toc" id="status">Status of this document</h2>
74 <!--status-->
76 <p>
77 The <a href="ChangeLog">list of changes made to this specification</a> is
78 available.
79 </p>
81 <h2 class="no-num no-toc" id="contents">Table of contents</h2>
82 <!--toc-->
85 <h2 id="introduction">Introduction</h2>
87 <p><em>This section is not normative.</em>
88 <p>
89 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.
90 </p>
92 <h2 id="transitions"><a id="transitions-">Transitions</a></h2>
93 <p>
94 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 transitions using new CSS properties. These properties are used to animate smoothly from the old state to the new state over time.
95 </p>
96 <p>
97 For example, suppose that transitions of one second have been defined on the <code class="property">'left'</code> and
98 <code class="property">'background-color'</code> 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.
99 </p>
100 <div class="figure">
101 <img src="transition1.png" alt="">
102 </div>
103 <p class="caption">
104 Transitions of <code class="property">'left'</code> and <code class="property">'background-color'</code>
105 </p>
106 <p>
107 Transitions are a presentational effect. The computed value of a property transitions over time from the old value to the new value. Therefore if a script queries the computed style of a property as it is transitioning, it will see an intermediate value that represents the current animated value of the property.
108 </p>
109 <p>
110 Only animatable CSS properties can be transitioned. See the table at the end of this document for a list
111 of properties that are animatable.
112 </p>
113 <p>
114 The transition for a property is defined using a number of new properties. For example:
115 </p>
116 <div class="example">
117 <p style="display:none">
118 Example(s):
119 </p>
120 <pre>
121 div {
122 transition-property: opacity;
123 transition-duration: 2s;
124 }
125 </pre>The above example defines a transition on the <code class="property">'opacity'</code> 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.
126 </div>
127 <p>
128 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:
129 </p>
130 <div class="example">
131 <p style="display:none">
132 Example(s):
133 </p>
134 <pre>
135 div {
136 transition-property: opacity, left;
137 transition-duration: 2s, 4s;
138 }
140 </pre>This will cause the <code class="property">'opacity'</code> property to transition over a period of two seconds and the left property to transition over a period of four seconds.
141 </div>
143 <p id="list-matching">
144 In the case where the lists of values in transition properties
145 do not have the same length, the length of the
146 'transition-property' list determines the number of items in
147 each list examined when starting transitions. The lists are
148 matched up from the first value: excess values at the end are
149 not used. If one of the other properties doesn't have enough
150 comma-separated values to match the number of values of
151 'transition-property', the UA must calculate its used value by
152 repeating the list of values until there are enough. This
153 truncation or repetition does not affect the computed value.
154 <span class="note">
155 Note: This is analogous to the behavior of the 'background-*'
156 properties, with 'background-image' analogous to
157 'transition-property'.
158 </span>
159 </p>
161 <div class="example">
162 <p style="display:none">
163 Example(s):
164 </p>
165 <pre>
166 div {
167 transition-property: opacity, left, top, width;
168 transition-duration: 2s, 1s;
169 }
170 </pre>The above example defines a transition on the <code class="property">'opacity'</code> property of 2 seconds duration, a
171 transition on the <code class="property">'left'</code> property of 1
172 second duration, a transition on the <code class="property">'top'</code> property of 2 seconds duration and a
173 transition on the <code class="property">'width'</code> property of 1
174 second duration.
176 </div>
178 <!-- ======================================================================================================= -->
179 <h3 id=transition-property-property><a id=the-transition-property-property->
180 The <code class="property">'transition-property'</code> Property
181 </a></h3>
182 <p>
183 The <code class="property">'transition-property'</code> property specifies the name of the CSS property to which the transition is applied.
184 </p>
185 <div class="issue">
186 We may ultimately want to support a keypath syntax for this property. A keypath syntax would enable different transitions to be specified for components of a property. For example the blur of a shadow could have a different transition than the color of a shadow.
187 </div>
188 <table class="propdef">
189 <tbody>
190 <tr>
191 <td>
192 <em>Name:</em>
193 </td>
194 <td>
195 <dfn id="transition-property">transition-property</dfn>
196 </td>
197 </tr>
198 <tr>
199 <td>
200 <em>Value:</em>
201 </td>
202 <td>
203 none | [ all | <IDENT> ] [ ',' [ all | <IDENT> ] ]*
204 </td>
205 </tr>
206 <tr>
207 <td>
208 <em>Initial:</em>
209 </td>
210 <td>
211 all
212 </td>
213 </tr>
214 <tr>
215 <td>
216 <em>Applies to:</em>
217 </td>
218 <td>
219 all elements, :before and :after pseudo elements
220 </td>
221 </tr>
222 <tr>
223 <td>
224 <em>Inherited:</em>
225 </td>
226 <td>
227 no
228 </td>
229 </tr>
230 <tr>
231 <td>
232 <em>Percentages:</em>
233 </td>
234 <td>
235 N/A
236 </td>
237 </tr>
238 <tr>
239 <td>
240 <em>Media:</em>
241 </td>
242 <td>
243 visual
244 </td>
245 </tr>
246 <tr>
247 <td>
248 <em>Computed value:</em>
249 </td>
250 <td>
251 Same as specified value.
252 </td>
253 </tr>
254 </tbody>
255 </table>
257 <p>
258 A value of 'none' means that no property will transition.
259 Otherwise, a list of properties to be transitioned, or the
260 keyword 'all' which indicates that all properties are to be
261 transitioned, is given.
262 </p>
264 <p>
265 If one of the identifiers listed is not a recognized property
266 name or is not an animatable property, the implementation must
267 still start transitions on the animatable properties in the
268 list using the duration, delay, and timing function at their
269 respective indices in the lists for 'transition-duration',
270 'transition-delay', and 'transition-timing-function'. In other
271 words, unrecognized or non-animatable properties must be kept in
272 the list to preserve the matching of indices.
273 </p>
274 <p class="issue">
275 Are 'none', 'inherit', and 'initial' allowed as items in
276 a list of identifiers (of length greater than one)?
277 </p>
278 <p>
279 For the keyword 'all', or if one of the identifiers listed is a
280 shorthand property, implementations must start transitions for
281 any of its longhand sub-properties that are animatable (or, for
282 'all', all animatable properties), using the duration, delay,
283 and timing function at the index corresponding to the shorthand.
284 </p>
285 <p>
286 If a property is specified multiple times in the value of
287 'transition-property' (either on its own, via a shorthand that
288 contains it, or via the 'all' value), then the transition that
289 starts uses the duration, delay, and timing function at the
290 index corresponding to the <em>last</em> item in the value of
291 'transition-property' that calls for animating that property.
292 </p>
293 <p class="note">
294 Note: The <code class="property">all</code> value and shorthand
295 properties work in similar ways, so the <code
296 class="property">all</code> value is just like a shorthand that
297 covers all properties.
298 </p>
300 <!-- ======================================================================================================= -->
301 <h3 id=transition-duration-property><a id=the-transition-duration-property->
302 The <code class="property">'transition-duration'</code> Property
303 </a></h3>
304 <p>
305 The <code class="property">'transition-duration'</code> property defines the length of time that a transition takes.
306 </p>
307 <table class="propdef">
308 <tbody>
309 <tr>
310 <td>
311 <em>Name:</em>
312 </td>
313 <td>
314 <dfn id="transition-duration">transition-duration</dfn>
315 </td>
316 </tr>
317 <tr>
318 <td>
319 <em>Value:</em>
320 </td>
321 <td>
322 <time> [, <time>]*
323 </td>
324 </tr>
325 <tr>
326 <td>
327 <em>Initial:</em>
328 </td>
329 <td>
330 0s
331 </td>
332 </tr>
333 <tr>
334 <td>
335 <em>Applies to:</em>
336 </td>
337 <td>
338 all elements, :before and :after pseudo elements
339 </td>
340 </tr>
341 <tr>
342 <td>
343 <em>Inherited:</em>
344 </td>
345 <td>
346 no
347 </td>
348 </tr>
349 <tr>
350 <td>
351 <em>Percentages:</em>
352 </td>
353 <td>
354 N/A
355 </td>
356 </tr>
357 <tr>
358 <td>
359 <em>Media:</em>
360 </td>
361 <td>
362 interactive
363 </td>
364 </tr>
365 <tr>
366 <td>
367 <em>Computed value:</em>
368 </td>
369 <td>
370 Same as specified value.
371 </td>
372 </tr>
373 </tbody>
374 </table>
375 <p>
376 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 <code class="property">transition-duration</code> is treated as '0s'.
377 </p>
379 <!-- =======================================================================================================
380 -->
382 <h3 id=transition-timing-function-property><a id=transition-timing-function_tag>
383 The <code class="property">'transition-timing-function'</code> Property
384 </a></h3>
385 <p>
386 The <code class="property">'transition-timing-function'</code> property
387 describes how the intermediate values used during a transition will be
388 calculated. It allows for a transition to change speed over its
389 duration. These effects are commonly called <em>easing</em> functions.
390 In either case, a mathematical function that provides a smooth curve is
391 used.
392 </p>
393 <p>
394 Timing functions are either defined as a stepping function or
395 a <a
396 href="http://en.wikipedia.org/wiki/B%C3%A9zier_curve#Cubic_B.C3.A9zier_curves">cubic
397 Bézier curve</a>.
398 The timing function takes as its input the current elapsed percentage of
399 the transition duration and outputs a percentage that determines how
400 close the transition is to its goal state.
401 </p>
402 <p>
403 A <a href="http://en.wikipedia.org/wiki/Step_function">stepping</a>
404 function is defined by a number that divides the domain of operation
405 into equally sized intervals. Each subsequent interval is a equal step
406 closer to the goal state. The function also specifies whether the
407 change in output percentage happens at the start or end of the
408 interval (in other words, if 0% on the input percentage is the point
409 of initial change).
410 </p>
411 <div class="figure">
412 <img src="step.png" alt="The step timing function splits
413 the function domain into a number of disjoint straight line
414 segments. steps(1, start) is a function whose
415 output value is 1 for all input values. steps(1, end) is a function whose
416 output value is 0 for all input values less than 1, and output
417 is 1 for the input value of 1. steps(3, start) is a function that
418 divides the input domain into three segments, each 1/3 in length,
419 and 1/3 above the previous segment, with the first segment starting
420 at 1/3. steps(3, end) is a function that
421 divides the input domain into three segments, each 1/3 in length,
422 and 1/3 above the previous segment, with the first segment starting
423 at 0.">
424 </div>
425 <p class="caption">
426 Step timing functions
427 </p>
428 <p>
429 A <a
430 href="http://en.wikipedia.org/wiki/B%C3%A9zier_curve#Cubic_B.C3.A9zier_curves">cubic
431 Bézier curve</a> is defined by four control points, P<sub>0</sub>
432 through P<sub>3</sub> (see Figure 1). P<sub>0</sub> and P<sub>3</sub>
433 are always set to (0,0) and (1,1). The <code class="property">'transition-timing-function'</code> property is used
434 to specify the values for points P<sub>1</sub> and P<sub>2</sub>. These
435 can be set to preset values using the keywords listed below, or can be
436 set to specific values using the <code class="css">'cubic-bezier'</code> function.
437 In the <code class="css">'cubic-bezier'</code> function, P<sub>1</sub> and
438 P<sub>2</sub> are each specified by both an X and Y value.
439 </p>
440 <div class="figure">
441 <img src="TimingFunction.png" alt="The Bézier timing function is a
442 smooth curve from point P0 = (0,0) to point P3 = (1,1). The
443 length and orientation of the line segment P0-P1 determines
444 the tangent and the curvature of the curve at P0 and the
445 line segment P2-P3 does the same at P3.">
446 </div>
447 <p class="caption">
448 Bézier Timing Function Control Points
449 </p>
450 <table class="propdef">
451 <tbody>
452 <tr>
453 <td>
454 <em>Name:</em>
455 </td>
456 <td>
457 <dfn id="transition-timing-function">transition-timing-function</dfn>
458 </td>
459 </tr>
460 <tr>
461 <td>
462 <em>Value:</em>
463 </td>
464 <td>
465 [ ease | linear | ease-in | ease-out | ease-in-out | step-start | step-end | steps(<integer>[, [ start | end ] ]?) | cubic-bezier(<number>, <number>, <number>, <number>) ] [, [ ease | linear | ease-in | ease-out | ease-in-out | step-start | step-end | steps(<number>[, [ start | end ] ]?) | cubic-bezier(<number>, <number>, <number>, <number>) ] ]*
466 </td>
467 </tr>
468 <tr>
469 <td>
470 <em>Initial:</em>
471 </td>
472 <td>
473 ease
474 </td>
475 </tr>
476 <tr>
477 <td>
478 <em>Applies to:</em>
479 </td>
480 <td>
481 all elements, :before and :after pseudo elements
482 </td>
483 </tr>
484 <tr>
485 <td>
486 <em>Inherited:</em>
487 </td>
488 <td>
489 no
490 </td>
491 </tr>
492 <tr>
493 <td>
494 <em>Percentages:</em>
495 </td>
496 <td>
497 N/A
498 </td>
499 </tr>
500 <tr>
501 <td>
502 <em>Media:</em>
503 </td>
504 <td>
505 interactive
506 </td>
507 </tr>
508 <tr>
509 <td>
510 <em>Computed value:</em>
511 </td>
512 <td>
513 Same as specified value.
514 </td>
515 </tr>
516 </tbody>
517 </table>
518 <p>
519 The timing functions have the following definitions.
520 </p>
521 <dl>
522 <dt>
523 ease
524 </dt>
525 <dd>
526 The ease function is equivalent to cubic-bezier(0.25, 0.1, 0.25, 1.0).
527 </dd>
528 <dt>
529 linear
530 </dt>
531 <dd>
532 The linear function is equivalent to cubic-bezier(0.0, 0.0, 1.0, 1.0).
533 </dd>
534 <dt>
535 ease-in
536 </dt>
537 <dd>
538 The ease-in function is equivalent to cubic-bezier(0.42, 0, 1.0, 1.0).
539 </dd>
540 <dt>
541 ease-out
542 </dt>
543 <dd>
544 The ease-out function is equivalent to cubic-bezier(0, 0, 0.58, 1.0).
545 </dd>
546 <dt>
547 ease-in-out
548 </dt>
549 <dd>
550 The ease-in-out function is equivalent to cubic-bezier(0.42, 0, 0.58, 1.0)
551 </dd>
552 <dt>
553 step-start
554 </dt>
555 <dd>
556 The step-start function is equivalent to steps(1, start).
557 </dd>
558 <dt>
559 step-end
560 </dt>
561 <dd>
562 The step-end function is equivalent to steps(1, end).
563 </dd>
564 <dt>
565 steps
566 </dt>
567 <dd>
568 Specifies a stepping function, described above, taking two
569 parameters. The first parameter specifies the number of intervals
570 in the function. It must be a positive integer (greater than 0).
571 The second parameter, which is optional, is
572 either the value 'start' or 'end', and specifies the point
573 at which the change of values occur within the interval.
574 If the second parameter is omitted, it is given the value 'end'.
575 </dd>
576 <dt>
577 cubic-bezier
578 </dt>
579 <dd>
580 Specifies a <a
581 href="http://en.wikipedia.org/wiki/B%C3%A9zier_curve">cubic-bezier
582 curve</a>. The four values specify points P<sub>1</sub> and
583 P<sub>2</sub> of the curve as (x1, y1, x2, y2). Both x values must be
584 in the range [0, 1] or the definition is invalid. The y values can
585 exceed this range.
586 </dd>
587 </dl><!-- ======================================================================================================= -->
588 <h3 id=transition-delay-property><a id=the-transition-delay-property->
589 The <code class="property">'transition-delay'</code> Property
590 </a></h3>
591 <p>
592 The <code class="property">'transition-delay'</code> property defines when the transition will start. It allows a transition to begin execution some some period of time from when it is applied. A <code class="property">'transition-delay'</code> 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.
593 </p>
594 <p>
595 If the value for <code class="property">'transition-delay'</code> 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 <code class="property">'transition-delay'</code>, the starting values are taken from the moment the property is changed.
596 </p>
597 <table class="propdef">
598 <tbody>
599 <tr>
600 <td>
601 <em>Name:</em>
602 </td>
603 <td>
604 <dfn id="transition-delay">transition-delay</dfn>
605 </td>
606 </tr>
607 <tr>
608 <td>
609 <em>Value:</em>
610 </td>
611 <td>
612 <time> [, <time>]*
613 </td>
614 </tr>
615 <tr>
616 <td>
617 <em>Initial:</em>
618 </td>
619 <td>
620 0s
621 </td>
622 </tr>
623 <tr>
624 <td>
625 <em>Applies to:</em>
626 </td>
627 <td>
628 all elements, :before and :after pseudo elements
629 </td>
630 </tr>
631 <tr>
632 <td>
633 <em>Inherited:</em>
634 </td>
635 <td>
636 no
637 </td>
638 </tr>
639 <tr>
640 <td>
641 <em>Percentages:</em>
642 </td>
643 <td>
644 N/A
645 </td>
646 </tr>
647 <tr>
648 <td>
649 <em>Media:</em>
650 </td>
651 <td>
652 interactive
653 </td>
654 </tr>
655 <tr>
656 <td>
657 <em>Computed value:</em>
658 </td>
659 <td>
660 Same as specified value.
661 </td>
662 </tr>
663 </tbody>
664 </table><!-- ======================================================================================================= -->
665 <h3 id=transition-shorthand-property><a id=the-transition-shorthand-property->
666 The <code class="property">'transition'</code> Shorthand Property
667 </a></h3>
668 <p>
669 The <code class="property">'transition'</code> shorthand property combines the four properties described above into a single property.
670 </p>
671 <p>
672 Note that order is important in this property. The first value that can be
673 parsed as a time is assigned to the transition-duration. The second value that
674 can be parsed as a time is assigned to transition-delay.
675 </p>
676 <p class="issue">
677 An alternative proposal is to accept the font shorthand approach of
678 using a "/" character between the values of the same type. e.g. 2s/4s would
679 mean a duration of 2 seconds and a delay of 4 seconds.
680 </p>
681 <table class="propdef">
682 <tbody>
683 <tr>
684 <td>
685 <em>Name:</em>
686 </td>
687 <td>
688 <dfn id="transition">transition</dfn>
689 </td>
690 </tr>
691 <tr>
692 <td>
693 <em>Value:</em>
694 </td>
695 <td>
696 [<'transition-property'> || <'transition-duration'> || <'transition-timing-function'> || <'transition-delay'> [, [<'transition-property'> || <'transition-duration'> || <'transition-timing-function'> || <'transition-delay'>]]*
697 </td>
698 </tr>
699 <tr>
700 <td>
701 <em>Initial:</em>
702 </td>
703 <td>
704 see individual properties
705 </td>
706 </tr>
707 <tr>
708 <td>
709 <em>Applies to:</em>
710 </td>
711 <td>
712 all elements, :before and :after pseudo elements
713 </td>
714 </tr>
715 <tr>
716 <td>
717 <em>Inherited:</em>
718 </td>
719 <td>
720 no
721 </td>
722 </tr>
723 <tr>
724 <td>
725 <em>Percentages:</em>
726 </td>
727 <td>
728 N/A
729 </td>
730 </tr>
731 <tr>
732 <td>
733 <em>Media:</em>
734 </td>
735 <td>
736 interactive
737 </td>
738 </tr>
739 <tr>
740 <td>
741 <em>Computed value:</em>
742 </td>
743 <td>
744 Same as specified value.
745 </td>
746 </tr>
747 </tbody>
748 </table>
750 <h2 id="starting">
751 Starting of transitions
752 </h2>
754 <p>
755 When the computed value of an animatable property changes,
756 implementations must decide what transitions to start based on
757 the values of the 'transition-property', 'transition-duration',
758 'transition-timing-function', and 'transition-delay' properties
759 at the time the animatable property would first have its new
760 computed value.
761 </p>
762 <div class="example" id="manual-reversing-example">
763 <p style="display:none">
764 Example(s):
765 </p>
766 <p>This provides a way for authors to specify different values
767 of the 'transition-*' properties for the “forward”
768 and “reverse” transitions (but see <a
769 href="#reversing">below</a> for special reversing behavior when
770 an <em>incomplete</em> transition is interrupted). Authors can
771 specify the value of 'transition-duration',
772 'transition-timing-function', or 'transition-delay' in the same
773 rule where they specify the value that triggers the transition,
774 or can change these properties at the same time as they change
775 the property that triggers the transition. Since it's the new
776 values of these 'transition-*' properties that affect the
777 transition, these values will be used for the transitions
778 <em>to</em> the associated transitioning values. For example:
779 </p>
780 <pre>li {
781 transition: background-color linear 1s;
782 background: blue;
783 }
784 li:hover {
785 background-color: green;
786 transition-duration: 2s; /* applies to the transition *to* the :hover state */
787 }</pre>
788 <p>
789 When a list item with these style rules enters the :hover
790 state, the computed 'transition-duration' at the time that
791 'background-color' would have its new value ('green') is '2s',
792 so the transition from 'blue' to 'green' takes 2 seconds.
793 However, when the list item leaves the :hover state, the
794 transition from 'green' to 'blue' takes 1 second.
795 </p>
796 </div>
798 <p>
799 When the computed value of a property changes, implementations
800 must start transitions based on the relevant item (see <a
801 href="#transition-property">the definition of
802 'transition-property'</a>) in the computed value of
803 'transition-property'.
804 Corresponding to this item there are
805 computed values of 'transition-duration' and 'transition-delay'
806 (see <a href="#list-matching">the rules on matching lists</a>).
807 Define the <dfn>combined duration</dfn> of the transition
808 as the sum of max('transition-duration', '0s') and 'transition-delay'.
809 When the combined duration is greater than '0s',
810 then a transition starts based on the values of
811 'transition-duration', 'transition-delay',
812 and 'transition-timing-function';
813 in other cases transitions do not occur.
814 </p>
816 <p>
817 Since this specification does not define
818 when computed values change, and thus what changes to
819 computed values are considered simultaneous,
820 authors should be aware that changing any of the transition
821 properties a small amount of time after making a change that
822 might transition can result in behavior that varies between
823 implementations, since the changes might be considered
824 simultaneous in some implementations but not others.
825 </p>
826 <p class="note">Say something about simultaneity</p>
828 <p>
829 Once the transition of a property has started, it must continue
830 running based on the original timing function, duration, and
831 delay, even if the 'transition-timing-function',
832 'transition-duration', or 'transition-delay' property changes
833 before the transition is complete. However, if the
834 'transition-property' property changes such that the transition
835 would not have started, the transition must stop (and the
836 property must immediately change to its final value).
837 </p>
839 <p>
840 Implementations must not start a transition when the computed
841 value of a property changes as a result of declarative animation
842 (as opposed to scripted animation).
843 </p>
845 <p>
846 Implementations also must not start a transition when the
847 computed value changes because it is inherited (directly or
848 indirectly) from another element that is transitioning the same
849 property.
850 </p>
852 <h2 id="reversing">
853 Automatically reversing interrupted transitions
854 </h2>
855 <p>
856 A common type of transition effect is when a running transition is
857 interrupted and the property is reset to its original value. An
858 example is a hover effect on an element, where the pointer enters and
859 exits the element before the effect has completed. If the outgoing and
860 incoming transitions are executed using their specified durations and
861 timing functions, the resulting effect can be distractingly
862 asymmetric. Instead, the expected behavior is that the new transition
863 should be the reverse of what has already executed.
864 </p>
866 <p>
867 If a running transition with duration T, executing so far for duration TE,
868 from state A, to state B, is interrupted by
869 a property change that would start a new transition back to state A, and
870 all the transition attributes are the same (duration, delay and timing function),
871 then the new transition must reverse the effect. The new transition must:
872 </p>
874 <ol>
875 <li>
876 Use the B and A states as its "from" and "to" states respectively. It
877 does not use the current value as its from state, due to the rules below.
878 </li>
879 <li>
880 Execute with the same duration T, but starting as if the transition had
881 already begun, without any transition delay, at the moment which would
882 cause the new transition to finish in TE from the moment of interruption. In other
883 words, the new transition will execute as if it started T-TE in the past.
884 </li>
885 <li>
886 Use a timing function that is the portion of the curve traversed up
887 to the moment of interruption, followed in the opposite direction (towards
888 the starting point). This will make the transition appear as if it
889 is playing backwards.
890 </li>
891 <li>
892 Ignore any transition delay.
893 </li>
894 </ol>
896 <p>
897 For example, suppose there is a transition with a duration of two
898 seconds. If this transition is interrupted after 0.5 seconds and the
899 property value assigned to the original value, then the new transition
900 effect will be the reverse of the original, as if it had begun
901 1.5 seconds in the past.
902 </p>
904 <p>
905 Note that by using the defined from and to states for the reversing
906 transition, it is also possible that it may reverse again, if
907 interrupted; for example, if the transition reversing to state A was
908 again interrupted by a property change to state B.
909 </p>
911 <p class="issue">Issue:
912 This introduces the concept of reversing a timing function,
913 which the spec has otherwise resisted doing, and also introduces
914 a discontinuity between transitions that have
915 almost completed (which get automatically reversed and thus have
916 their timing function reversed) and transitions that have fully
917 completed (where the reversal doesn't lead to the timing
918 function being reversed). An alternative proposal that avoids
919 this is to follow the normal timing function algorithm, except
920 multiply the duration (and also shorten any negative delay) by
921 the (output) value of the transition timing function of the
922 incomplete transition at the time it was interrupted, and, to
923 account for multiple reverses in sequence, to divide by the
924 shortening applied to the transition being interrupted. For
925 more details see this thread:
926 <a href="http://lists.w3.org/Archives/Public/www-style/2009Nov/thread.html#msg302">November 2009 part</a>,
927 <a href="http://lists.w3.org/Archives/Public/www-style/2009Dec/thread.html#msg319">December 2009 part</a>,
928 <a href="http://lists.w3.org/Archives/Public/www-style/2010Jan/thread.html#msg136">January 2010 part</a>.
929 </p>
931 <h2 id=transition-events><a id=transition-events->
932 Transition Events
933 </a></h2>
934 <p>
935 The completion of a CSS Transition generates a corresponding <a href="http://www.w3.org/TR/DOM-Level-2-Events/events.html">DOM Event</a>.
936 An event is fired for each property that undergoes a transition.
937 This allows a content developer to perform actions that synchronize
938 with the completion of a transition.
939 </p>
940 <p>
941 Each event provides the name of the property the transition is
942 associated with as well as the duration of the transition.
943 </p>
944 <dl>
945 <dt>
946 <b>Interface <i><a id="Events-TransitionEvent" name='Events-TransitionEvent'>TransitionEvent</a></i></b>
947 </dt>
948 <dd>
949 <p>
950 The <code>TransitionEvent</code> interface provides specific contextual information associated with transitions.
951 </p>
952 <dl>
953 <dt>
954 <b>IDL Definition</b>
955 </dt>
956 <dd>
957 <div class='idl-code'>
958 <pre>
959 interface TransitionEvent : Event {
960 readonly attribute DOMString propertyName;
961 readonly attribute float elapsedTime;
962 readonly attribute DOMString pseudoElement;
963 void initTransitionEvent(in DOMString typeArg,
964 in boolean canBubbleArg,
965 in boolean cancelableArg,
966 in DOMString propertyNameArg,
967 in float elapsedTimeArg,
968 in DOMString pseudoElementArg);
969 };
970 </pre>
971 </div>
972 </dd>
973 <dt>
974 <b>Attributes</b>
975 </dt>
976 <dd>
977 <dl>
978 <dt>
979 <code class='attribute-name'><a id="Events-TransitionEvent-propertyName" name='Events-TransitionEvent-propertyName'>propertyName</a></code> of type <code>DOMString</code>, readonly
980 </dt>
981 <dd>
982 The name of the CSS property associated with the transition.
983 </dd>
984 </dl>
985 <dl>
986 <dt>
987 <code class='attribute-name'><a id="Events-TransitionEvent-elapsedTime" name='Events-TransitionEvent-elapsedTime'>elapsedTime</a></code> of type <code>float</code>, readonly
988 </dt>
989 <dd>
990 The amount of time the transition has been running, in seconds, when this event fired. Note that this value is not affected by the value of <code class="property">transition-delay</code>.
991 </dd>
992 </dl>
993 <dl>
994 <dt>
995 <code class='attribute-name'><a id="Events-TransitionEvent-pseudoElement" name='Events-TransitionEvent-pseudoElement'>pseudoElement</a></code> of type <code>DOMString</code>, readonly
996 </dt>
997 <dd>
998 The name (beginning with two colons) of the CSS
999 pseudo-element on which the transition occured (in
1000 which case the target of the event is that
1001 pseudo-element's corresponding element), or the empty
1002 string if the transition occurred on an element (which
1003 means the target of the event is that element).
1004 </dd>
1005 </dl>
1006 </dd>
1007 <dt>
1008 <b>Methods</b>
1009 </dt>
1010 <dd>
1011 <dl>
1012 <dt>
1013 <code class='method-name'><a id="Events-TransitionEvent-initTransitionEvent" name='Events-TransitionEvent-initTransitionEvent'>initTransitionEvent</a></code>
1014 </dt>
1015 <dd>
1016 <div class='method'>
1017 The <code>initTransitionEvent</code> method is used to
1018 initialize the value of a <code>TransitionEvent</code>
1019 created through the <a
1020 href='http://www.w3.org/TR/DOM-Level-2-Events/events.html#Events-DocumentEvent'><code>DocumentEvent</code></a>
1021 interface. This method may only be called before the
1022 <code>TransitionEvent</code> has been dispatched via the
1023 <code>dispatchEvent</code> method, though it may be called
1024 multiple times during that phase if necessary. If called
1025 multiple times, the final invocation takes precedence.
1026 <p class="issue">Should new events being created still
1027 have init*Event methods?</p>
1028 <div class='parameters'>
1029 <b>Parameters</b>
1030 <div class='paramtable'>
1031 <dl>
1032 <dt>
1033 <code class='parameter-name'>typeArg</code> of type <code>DOMString</code>
1034 </dt>
1035 <dd>
1036 Specifies the event type.<br>
1037 </dd>
1038 <dt>
1039 <code class='parameter-name'>canBubbleArg</code> of type <code>boolean</code>
1040 </dt>
1041 <dd>
1042 Specifies whether or not the event can bubble.<br>
1043 </dd>
1044 <dt>
1045 <code class='parameter-name'>cancelableArg</code> of type <code>boolean</code>
1046 </dt>
1047 <dd>
1048 Specifies whether or not the event's default action can be prevented. Since a TransitionEvent
1049 is purely for notification, there is no default action.<br>
1050 </dd>
1051 <dt>
1052 <code class='parameter-name'>propertyNameArg</code> of type <code>DOMString</code>
1053 </dt>
1054 <dd>
1055 Specifies the name of the property associated with the <a href='http://www.w3.org/TR/DOM-Level-2-Events/events.html#Events-Event'><code>Event</code></a>.
1056 (See the <a href="#Events-TransitionEvent-propertyName">propertyName</a> attribute.)
1057 </dd>
1058 <dt>
1059 <code class='parameter-name'>elapsedTimeArg</code> of type <code>float</code>
1060 </dt>
1061 <dd>
1062 Specifies the amount of time, in seconds, the transition has been running at the time of initialization.
1063 (See the <a href="#Events-TransitionEvent-elapsedTime">elapsedTime</a> attribute.)
1064 </dd>
1065 <dt>
1066 <code class='parameter-name'>pseudoElementArg</code> of type <code>DOMString</code>
1067 </dt>
1068 <dd>
1069 Specifies the pseudo-element on which the
1070 transition occurred.
1071 (See the <a href="#Events-TransitionEvent-pseudoElement">pseudoElement</a> attribute.)
1072 <span class="issue">Does adding this additional argument create any compatibility problems?</span>
1073 </dd>
1074 </dl>
1075 </div>
1076 </div><!-- parameters -->
1077 <div>
1078 <b>No Return Value</b>
1079 </div>
1080 <div>
1081 <b>No Exceptions</b>
1082 </div>
1083 </div><!-- method -->
1084 </dd>
1085 </dl>
1086 </dd>
1087 </dl>
1088 </dd>
1089 </dl>
1090 <p>
1091 There is one type of transition event available.
1092 </p>
1093 <dl>
1094 <dt>
1095 <b>transitionend</b>
1096 </dt>
1097 <dd>
1098 The 'transitionend' event occurs at the completion of the transition. In the
1099 case where a transition is removed before completion, such as if the
1100 transition-property is removed, then the event will not fire.
1101 <ul>
1102 <li>Bubbles: Yes
1103 </li>
1104 <li>Cancelable: Yes
1105 </li>
1106 <li>Context Info: propertyName, elapsedTime
1107 </li>
1108 </ul>
1109 </dd>
1110 </dl>
1112 <h2 id="animatable-types"><a id=animation-of-property-types->
1113 Animation of property types
1114 </a></h2>
1116 <p>
1117 The following describes how each property type undergoes transition or
1118 animation.
1119 </p>
1121 <ul>
1122 <li>
1123 <strong>color</strong>: interpolated via red, green, blue and alpha
1124 components (treating each as a number, see below).
1125 <div class="issue">Issue: Are the colors interpolated in
1126 premultiplied space or non-premultiplied space?</div>
1127 </li>
1128 <li>
1129 <strong>length</strong>: interpolated as real numbers.
1130 </li>
1131 <li>
1132 <strong>percentage</strong>: interpolated as real numbers.
1133 </li>
1134 <li>
1135 <strong>integer</strong>: interpolated via discrete steps (whole
1136 numbers). The interpolation happens in real number space and is
1137 converted to an integer using <code>floor()</code>.
1138 <span class="issue">
1139 This floor behavior is inconsistent with SMIL Animation /
1140 SVG Animation.
1141 </span>
1142 </li>
1143 <li>
1144 <strong>font weight</strong>: interpolated via discrete steps
1145 (multiples of 100). The interpolation happens in real number
1146 space and is converted to an integer by rounding to the
1147 nearest multiple of 100.
1148 <span class="issue">
1149 This round-to-nearest behavior is inconsistent with the
1150 floor behavior used for integer types, but probably should
1151 be consistent (one way or the other).
1152 </span>
1153 </li>
1154 <li>
1155 <strong>number</strong>: interpolated as real (floating point)
1156 numbers.
1157 </li>
1158 <li>
1159 <strong>transform list</strong>: see
1160 CSS Transforms specification [[!CSS3-TRANSFORMS]].
1161 </li>
1162 <li>
1163 <strong>rectangle</strong>: interpolated via the x, y,
1164 width and height components (treating each as a number).
1165 </li>
1166 <li>
1167 <strong>visibility</strong>: if one of the values is
1168 'visible', interpolated as a discrete step where values of the
1169 timing function between 0 and 1 map to 'visible' and other
1170 values of the timing function (which occur only at the
1171 start/end of the transition or as a result of 'cubic-bezier()'
1172 functions with Y values outside of [0, 1]) map to the closer
1173 endpoint; if neither value is 'visible' then not interpolable.
1174 </li>
1175 <li>
1176 <strong>shadow</strong>: interpolated via the color, x, y
1177 and blur components (treating them as color and numbers where
1178 appropriate). In the case where there are lists of shadows,
1179 the shorter list is padded at the end with shadows whose
1180 color is transparent and all lengths (x, y, blur) are 0.
1181 </li>
1182 <li>
1183 <strong>gradient</strong>: interpolated via the
1184 positions and colors of each stop. They must have the same type
1185 (radial or linear) and same number of stops in order to be animated.
1186 <span class="note">Note: [[CSS3-IMAGES]] may extend this
1187 definition.</span>
1188 </li>
1189 <li>
1190 <strong>paint server</strong> (SVG): interpolation is only supported
1191 between: gradient to gradient and color to color. They then
1192 work as above.
1193 </li>
1194 <li>
1195 <strong>list of above types</strong>: If the lists have the
1196 same number of items, each item in the list is interpolated using the
1197 rules above. Otherwise the interpolation is determined by the property
1198 rules. If the property extends its list by repeating values, then this
1199 repeated form will be used in the interpolation (<code class="property">'background-position'</code>
1200 is an example of a property that would transition between lists of different lengths). If
1201 the property does not allow extending its list, then no interpolation
1202 will occur.
1203 </li>
1204 <li>
1205 <strong>a shorthand property</strong>: If any part of a
1206 shorthand can be animated, then interpolation is performed as
1207 if those animatable properties were individually specified.
1208 </li>
1209 </ul>
1211 <p class="issue">Issue: Need to describe handling of out-of-range
1212 values that can result from cubic-bezier(). Clamping values to the
1213 allowed range is probably the best solution.</p>
1215 <h2 id=animatable-properties><a id=animatable-properties->
1216 Animatable properties
1217 </a></h2>
1219 <!--
1220 As resolved in
1221 http://lists.w3.org/Archives/Public/www-style/2011Sep/0497.html
1222 -->
1223 <p>For properties that exist at the time this specification was
1224 developed, this specification defines whether and how they are
1225 animated. However, future CSS specifications may define
1226 additional properties, additional values for existing properties,
1227 or additional animation behavior of existing values. In order to
1228 describe new animation behaviors and to have the definition of
1229 animation behavior in a more appropriate location, future CSS
1230 specifications should include an "Animatable:" line in the summary
1231 of the property's definition (in addition to the other lines
1232 described in [[CSS21]], <a
1233 href="http://www.w3.org/TR/CSS21/about.html#property-defs">section
1234 1.4.2</a>). This line should say "no" to indicate that a property
1235 cannot be animated or should reference an animation behavior
1236 (which may be one of the behaviors in the <a
1237 href="#animation-of-property-types-">Animation of property
1238 types</a> section above, or may be a new behavior) to define how
1239 the property animates. Such definitions override those given in
1240 this specification.</p>
1242 <h3 id=animatable-css><a id=properties-from-css->
1243 Properties from CSS
1244 </a></h3>
1246 <table class="animatable-properties">
1247 <tr>
1248 <th>Property Name</th>
1249 <th>Type</th>
1250 </tr>
1251 <tr>
1252 <td>background-color</td><td>color</tr>
1253 <tr>
1254 <td>background-position</td><td>percentage, length</td>
1255 </tr>
1256 <tr>
1257 <td>border-bottom-color</td><td>color</td>
1258 </tr>
1259 <tr>
1260 <td>border-bottom-width</td><td>length</td>
1261 </tr>
1262 <tr>
1263 <td>border-left-color</td><td>color</td>
1264 </tr>
1265 <tr>
1266 <td>border-left-width</td><td>length</td>
1267 </tr>
1268 <tr>
1269 <td>border-right-color</td><td>color</td>
1270 </tr>
1271 <tr>
1272 <td>border-right-width</td><td>length</td>
1273 </tr>
1274 <tr>
1275 <td>border-spacing</td><td>length</td>
1276 </tr>
1277 <tr>
1278 <td>border-top-color</td><td>color</td>
1279 </tr>
1280 <tr>
1281 <td>border-top-width</td><td>length</td>
1282 </tr>
1283 <tr>
1284 <td>bottom</td><td>length, percentage</td>
1285 </tr>
1286 <tr>
1287 <td>clip</td><td>rectangle</td>
1288 </tr>
1289 <tr>
1290 <td>color</td><td>color</td>
1291 </tr>
1292 <tr>
1293 <td>crop <span class="issue">css3-content will likely advance slower than this specification, in which case this definition should move there</span></td><td>rectangle</td>
1294 </tr>
1295 <tr>
1296 <td>font-size</td><td>length, percentage</td>
1297 </tr>
1298 <tr>
1299 <td>font-weight</td><td>font weight</td>
1300 </tr>
1301 <tr>
1302 <td>height</td><td>length, percentage</td>
1303 </tr>
1304 <tr>
1305 <td>left</td><td>length, percentage</td>
1306 </tr>
1307 <tr>
1308 <td>letter-spacing</td><td>length</td>
1309 </tr>
1310 <tr>
1311 <td>line-height</td><td>number, length, percentage</td>
1312 </tr>
1313 <tr>
1314 <td>margin-bottom</td><td>length</td>
1315 </tr>
1316 <tr>
1317 <td>margin-left</td><td>length</td>
1318 </tr>
1319 <tr>
1320 <td>margin-right</td><td>length</td>
1321 </tr>
1322 <tr>
1323 <td>margin-top</td><td>length</td>
1324 </tr>
1325 <tr>
1326 <td>max-height</td><td>length, percentage</td>
1327 </tr>
1328 <tr>
1329 <td>max-width</td><td>length, percentage </td>
1330 </tr>
1331 <tr>
1332 <td>min-height</td><td>length, percentage</td>
1333 </tr>
1334 <tr>
1335 <td>min-width</td><td>length, percentage</td>
1336 </tr>
1337 <tr>
1338 <td>opacity</td><td>number</td>
1339 </tr>
1340 <tr>
1341 <td>outline-color</td><td>color</td>
1342 </tr>
1343 <tr>
1344 <td>outline-offset</td><td>integer</td>
1345 </tr>
1346 <tr>
1347 <td>outline-width</td><td>length</td>
1348 </tr>
1349 <tr>
1350 <td>padding-bottom</td><td>length</td>
1351 </tr>
1352 <tr>
1353 <td>padding-left</td><td>length</td>
1354 </tr>
1355 <tr>
1356 <td>padding-right</td><td>length</td>
1357 </tr>
1358 <tr>
1359 <td>padding-top</td><td>length</td>
1360 </tr>
1361 <tr>
1362 <td>right</td><td>length, percentage</td>
1363 </tr>
1364 <tr>
1365 <td>text-indent</td><td>length, percentage</td>
1366 </tr>
1367 <tr>
1368 <td>text-shadow</td><td>shadow</td>
1369 </tr>
1370 <tr>
1371 <td>top</td><td>length, percentage</td>
1372 </tr>
1373 <tr>
1374 <td>vertical-align</td><td>length, percentage</td>
1375 </tr>
1376 <tr>
1377 <td>visibility</td><td>visibility</td>
1378 </tr>
1379 <tr>
1380 <td>width</td><td>length, percentage</td>
1381 </tr>
1382 <tr>
1383 <td>word-spacing</td><td>length, percentage</td>
1384 </tr>
1385 <tr>
1386 <td>z-index</td><td>integer</td>
1387 </tr>
1388 </table>
1390 <p class="issue">
1391 This list omits the following properties that Gecko can animate, and
1392 which likely should be included:
1393 background-size,
1394 border-*-radius,
1395 box-shadow,
1396 column-count,
1397 column-gap,
1398 column-rule-color,
1399 column-rule-width,
1400 column-width,
1401 font-size-adjust,
1402 font-stretch,
1403 marker-offset,
1404 text-decoration-color,
1405 transform,
1406 transform-origin.
1407 </p>
1409 <h3 id=animatable-svg><a id=properties-from-svg->
1410 Properties from SVG
1411 </a></h3>
1413 <p>
1414 All properties defined as animatable in the SVG specification, provided
1415 they are one of the property types listed above.
1416 </p>
1418 <!-- <table>
1419 <tr>
1420 <th>Property Name</th><th>Type</th>
1421 </tr>
1422 <tr>
1423 <td>stop-color</td><td>color</td>
1424 </tr>
1425 <tr>
1426 <td>stop-opacity</td><td>float</td>
1427 </tr>
1428 <tr>
1429 <td>fill</td><td>paint server</td>
1430 </tr>
1431 <tr>
1432 <td>fill-opacity</td><td>float</td>
1433 </tr>
1434 <tr>
1435 <td>stroke</td><td>paint server</td>
1436 </tr>
1437 <tr>
1438 <td>stroke-dasharray</td><td>list of numbers</td>
1439 </tr>
1440 <tr>
1441 <td>stroke-dashoffset</td><td>number</td>
1442 </tr>
1443 <tr>
1444 <td>stroke-miterlimit</td><td>number</td>
1445 </tr>
1446 <tr>
1447 <td>stroke-opacity</td><td>float</td>
1448 </tr>
1449 <tr>
1450 <td>stroke-width</td><td>float</td>
1451 </tr>
1452 <tr>
1453 <td>viewport-fill</td><td>color</td>
1454 </tr>
1455 <tr>
1456 <td>viewport-fill-opacity</td><td>color</td>
1457 </tr>
1458 </table> -->
1460 <h2 id="acknowledgments">Acknowledgments</h2>
1462 <p>Thanks especially to the feedback from
1463 Tab Atkins,
1464 Aryeh Gregor,
1465 Vincent Hardy,
1466 Cameron McCormack,
1467 Alex Mogilevsky,
1468 and all the rest of the
1469 <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.</p>
1471 <h2 id="references">References</h2>
1473 <h3 class="no-num" id=normative-references>Normative references</h3>
1474 <!--normative-->
1476 <h3 class="no-num" id=other-references>Other references</h3>
1477 <!--informative-->
1481 <h2 class="no-num" id=property-index>Property index</h2>
1482 <!-- properties -->
1486 <h2 class="no-num" id="index">Index</h2>
1487 <!--index-->
1489 </body>
1490 </html>
1491 <!-- Keep this comment at the end of the file
1492 Local variables:
1493 mode: sgml
1494 sgml-default-doctype-name:"html"
1495 sgml-minimize-attributes:t
1496 End:
1497 -->