Added web-animations-js to third_party/web-animations-js.

third_party/web-animations-js/README.md

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+[![Build Status](https://travis-ci.org/web-animations/web-animations-js.png?branch=master)](https://travis-ci.org/web-animations/web-animations-js)
+
+Latest specification at http://w3c.github.io/web-animations/.
+
+## Learn the tech
+
+### Why Web Animations?
+
+Four animation-related specifications already exist on the web platform: [CSS Transitions](http://dev.w3.org/csswg/css-transitions/),
+[CSS Animations](http://dev.w3.org/csswg/css-animations/), [SVG Animations](http://www.w3.org/TR/SVG/animate.html) / [SMIL](http://www.w3.org/TR/2001/REC-smil-animation-20010904/), and `requestAnimationFrame()`. However:
+
+- *CSS Transitions / CSS Animations are not very expressive* - animations can't
+be composed, or sequenced, or even reliably run in parallel; and animations can't be tweaked from script.
+- *SVG Animations are very expressive, but also very complicated*. SVG Animations
+can't be applied to HTML content.
+- *`requestAnimationFrame()` is not a declarative approach* - it requires the use
+of the main thread, and will therefore jank if the main thread is busy.
+
+Web Animations is a new specification for animated content on the web. It's being
+developed as a W3C specification as part of the CSS and SVG working groups. It aims
+to address the deficiencies inherent in these four specifications. Web Animations also aims to replace the underlying implementations of CSS Transitions, CSS Animations and SVG Animations, so that:
+
+- The code cost of supporting animations on the web is reduced.
+- The various animations specifications are interoperable.
+- Spec authors and browser vendors have a single place to experiment with new animation innovations to improve the Web for the future.
+
+### Basic usage
+
+Here's a simple example of an animation that scales and changes the opacity of
+a `<div>` over 0.5 seconds. The animation alternates producing a pulsing effect.
+
+    <div class="pulse" style="width:150px;">Hello world!</div>
+    <script>
+      var elem = document.querySelector('.pulse');
+      var player = document.timeline.play(new Animation(elem, [
+          {opacity: "0.5", transform: "scale(0.5)"}, 
+          {opacity: "1.0", transform: "scale(1)"}
+        ],
+        {
+          direction: "alternate", duration: 500, iterations: Infinity
+        }));
+    </script>
+
+### The animation model
+
+The Web Animations model is a description of an engine for animation content on the web. The engine is sufficiently powerful to support CSS Transitions, CSS Animations and SVG Animations.
+
+Web Animations also exposes a JS API to the model. This API defines a number of
+new interfaces that are exposed to JavaScript. We'll go through some of the more
+important ones here: Animations, AnimationEffects, TimingDictionaries, TimingGroups, and AnimationPlayers.
+
+An `Animation` object defines a single animation effect that applies to a single element target. For example:
+
+    var animation = new Animation(targetElement,
+        [{left: '0px'}, {left: '100px'}], 2000);
+
+Here, the target element's "left" CSS property is modified smoothly from `0px` to `100px` over 2 seconds.
+
+### Specifying animation effects
+
+An `AnimationEffect` object controls which CSS properties and SVG attributes are
+modified by an animation, and the values that those properties and attributes
+vary between. AnimationEffect objects also control whether the effect replaces
+or adds to the underlying value.
+
+There are three major kinds of effects: `KeyframeEffect`, `MotionPathEffect`, and `EffectCallback`.
+
+#### Animating between keyframes
+
+A `KeyframeEffect` controls one or more properties/attributes by linearly
+interpolating values between specified keyframes. KeyframeEffects are usually
+defined by specifying the keyframe offset and the property-value pair in a
+dictionary:
+
+    [
+      {offset: 0.2, left: "35px"},
+      {offset: 0.6, left: "50px"},
+      {offset: 0.9, left: "70px"},
+    ]
+
+If the offset is not specified, keyframes are evenly distributed at offsets
+between 0 and 1.
+
+    [{left: "35px"}, {left: "50px"}, {left: "70px"}]
+
+See the [specification](http://www.w3.org/TR/web-animations/#keyframe-animation-effects) for the details
+of the keyframe distribution procedure, and how KeyframeEffects are
+evaluated at offsets outside those specified by the keyframes.
+
+#### Animating along paths
+
+A `MotionPathEffect` allows elements to be animated along SVG-style paths. For example:
+
+    <svg xmlns="http://www.w3.org/2000/svg" version="1.1">
+      <defs>
+        <path id=path d="M 100,100 a 75,75 0 1,0 150,0 a 75,75 0 1,0 -150,0"/>
+      </defs>
+    </svg>
+    <script>
+      var animFunc = new MotionPathEffect(document.querySelector('#path').pathSegList);
+      var animation = new Animation(targetElement, animFunc, 2000);
+    </script>
+
+#### Custom animation effects
+
+An `EffectCallback` allows animations to generate call-outs to JavaScript
+rather than manipulating properties directly. Please see the
+[specification](http://www.w3.org/TR/web-animations/#custom-effects) for more details on this
+feature.
+
+### Sequencing and synchronizing animations
+
+Two different types of TimingGroups (`AnimationGroup` and `AnimationSequence`) allow animations to be synchronized and sequenced.
+
+To play a list of animations in parallel:
+
+    var animationGroup = new AnimationGroup([new Animation(...), new Animation(...)]);
+
+To play a list in sequence:
+
+    var animationSequence = new AnimationSequence([new Animation(...), new Animation(...)]);
+
+Because `Animation`, `AnimationGroup`, `AnimationSequence` are all TimedItems, groups can be nested:
+
+    var animationGroup = new AnimationGroup([
+      new AnimationSequence([
+        new Animation(...),
+        new Animation(...),
+      ]),
+      new Animation(...)
+    ]);
+
+Groups also take an optional TimingDictionary parameter (see below), which among other things allows iteration and timing functions to apply at the group level:
+
+    var animationGroup = new AnimationGroup([new Animation(...), new Animation(...)], {iterations: 4});
+
+### Controlling the animation timing
+
+TimingDictionaries are used to control the internal timing of an animation (players control how an animation progresses relative to document time). TimingDictionaries have several properties that can be tweaked:
+
+- **duration**: the duration of a single iteration of the animation
+- **iterations**: the number of iterations of the animation that will be played (fractional iterations are allowed)
+- **iterationStart**: the start offset of the first iteration
+- **fill**: whether the animation has effect before starting the first iteration and/or after finishing the final iteration
+- **delay**: the time between the animation's start time and the first animation effect of the animation
+- **playbackRate**: the rate at which the animation progresses relative to external time
+- **direction**: the direction in which successive iterations of the animation play back
+- **easing**: fine-grained control over how external time impacts an animation across the total active duration of the animation.
+
+The values provided within TimingDictionaries combine with the animation hierarchy
+to generate concrete start and end values for animation iterations, animation
+backwards fills, and animation forwards fills. There are a few simple rules which govern this:
+
+- Animations never extend beyond the start or end values of their parent iteration.
+- Animations only fill beyond their parent iteration if:
+    - the relevant fill value is selected for the animation;
+    - the matching fill value is selected for the parent; and
+    - this is the first parent iteration (for `fill: 'backwards'`) or last parent iteration (for `fill: 'forwards'`)
+- Missing `duration` values for TimingGroups are generated based on the calculated durations of the child animations.
+
+The following example illustrates these rules:
+
+    var animationGroup = new AnimationGroup([
+      new AnimationSequence([
+        new Animation(..., {duration: 3000}),
+        new Animation(..., {duration: 5000, fill: 'both'})
+      ], {duration: 6000, delay: 3000, fill: 'none'}),
+      new Animation(..., {duration: 8000, fill: 'forwards'})
+    ], {iterations: 2, fill: 'forwards'});
+
+In this example:
+
+- The `AnimationSequence` has an explicit `duration` of 6 seconds, and so the
+second child animation will only play for the first 3 of its 5 second duration
+- The `AnimationGroup` has no explicit duration, and will be provided with a
+calculated duration of the max (`duration + delay`) of its children - in this case 9 seconds.
+- Although `fill: "both"` is specified for the second `Animation` within the `AnimationSequence`, the `AnimationSequence` itself has a `fill` of "none". Hence, as the animation ends right at the end of the `AnimationSequence`, the animation will only fill backwards, and only up until the boundary of the `AnimationSequence` (i.e. 3 seconds after the start of the `AnimationGroup`).
+- The `Animation` inside the `AnimationGroup` and the `AnimationGroup` are both `fill: "forwards"`. Therefore the animation will fill forward in two places: 
+    - from 8 seconds after the `AnimationGroup` starts until the second iteration of the `AnimationGroup` starts (i.e. for 1 second)
+    - from 17 seconds after the `AnimationGroup` starts, extending forward indefinitely.
+
+### Playing Animations
+
+In order to play an `Animation` or `TimingGroup`, an `AnimationPlayer` must be constructed:
+
+    var player = document.timeline.play(myAnimation);
+
+AnimationPlayers provide complete control the start time and current playback head of their attached animation. However, players can't modify any internal details of an animation.
+
+AnimationPlayers can be used to pause, seek, reverse, or modify the playback rate of an animation.
+
+`document.timeline.currentTime` is a timeline's global time. It gives the number
+of seconds since the document fired its load event.
+
+## Polyfill details
+
+### Getting started
+
+Include `web-animations.js` in your project:
+
+    <script src="web-animations-js/web-animations.js"></script> 
+
+### Polyfill notes
+
+#### Prefix handling
+
+In order to work in as many browsers as feasible, we have decided to take the
+following approach to prefix handling:
+
+- the polyfill will automatically detect the correctly prefixed name to use when
+writing animated properties back to the platform.
+- where possible, the polyfill will *only* accept unprefixed versions of experimental features. For example:
+
+        var animation = new Animation(elem, {"transform": "translate(100px, 100px)"}, 2000);
+
+  will work in all browsers that implement a conforming version of `transform`, but
+
+        var animation =  new Animation(elem, {"-webkit-transform": "translate(100px, 100px)"}, 2000);
+    
+  will not work anywhere.
+
+#### Experimental features
+
+When the polyfill requires features to implement functionality that is not inherently specified using those
+features (for example, CSS `calc()` is required in order to implement merging between lengths with different units) 
+then the polyfill will provide a console warning in browsers where these features are absent.
+
+## Tools & testing
+
+For running tests or building minified files, consult the
+[tooling information](http://www.polymer-project.org/resources/tooling-strategy.html).
+
+## Breaking changes
+
+When we make a potentially breaking change to the polyfill's API surface (like a rename) we'll continue supporting the old version, deprecated, for three months and ensure that there are console warnings that a change is pending. After three months, the old version of the API surface (e.g. the old version of a function name) will be removed. If you see deprecation warnings you can't avoid it by not updating.
+
+We also announce anything that isn't a bug fix on web-animations-changes@googlegroups.com (https://groups.google.com/forum/#!forum/web-animations-changes).