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AMP

amp-animation

Description

Defines and displays an animation.

Required Scripts

<script async custom-element="amp-animation" src="https://cdn.ampproject.org/v0/amp-animation-0.1.js"></script>

Supported Layouts

Usage

The amp-animation component defines and runs custom animations and effects. It relies on the Web Animations API.

An amp-animation component defines animations in a JSON structure. The top-level section defines the overarching animation by declaring target element(s), execution conditions, timing properties and keyframes effect. The overarching process can contain any arbitrary number of animation parts defined within the animations array. Animation parts in the animation's array may have their own target elements, execution conditions, timing properties, and keyframes effects.

<amp-animation layout="nodisplay">
  <script type="application/json">
    {
      "selector": "#target-id",
      "duration": "1s",
      "iterations": "4",
      "fill": "both",
      "direction": "alternate",
      "animations": [
        {
          "selector": ".target-class",
          "easing": "cubic-bezier(0,0,.21,1)",
          "keyframes": {
            "transform": "rotate(20deg)"
          }
        },
        {
          "delay": "2s",
          "easing": "cubic-bezier(0,0,.21,1)",
          "keyframes": {
            "transform": "rotate(30deg)"
          }
        }
      ]
    }
  </script>
</amp-animation>

If the animation uses a single element and a single keyframes effect, the configuration is valid as a single animation definition.

<amp-animation layout="nodisplay">
  <script type="application/json">
    {
      "selector": "#target-id",
      "duration": "1s",
      "keyframes": {"opacity": 1}
    }
  </script>
</amp-animation>

If the animation uses multiple elements, but does not have a top-level animation, the configuration is valid as an array of definitions.

<amp-animation layout="nodisplay">
  <script type="application/json">
    [
      {
        "selector": ".target1",
        "duration": 1000,
        "keyframes": {"opacity": 1}
      },
      {
        "selector": ".target2",
        "duration": 600,
        "delay": 400,
        "keyframes": {"transform": "scale(2)"}
      }
    ]
  </script>
</amp-animation>

Trigger the start of one or multiple animations via the trigger attribute or an action.

You may place amp-animation controlled via actions anywhere in the DOM. If the animation contains trigger="visibility" it will be triggered when the parent element comes into the viewport, and paused when it leaves the viewport.

Defining effects

Keyframes

You must declare effects as keyframes to apply animations using amp-animations.

You may specify keyframes in amp-animation in the same way as defined in MDN's Keyframe Formats. You may also reference the @keyframes name defined as CSS within the <style amp-custom> or <style amp-keyframes> tag.

Some typical examples of keyframes definitions are below.

Shorthand object-form "to" format specifies the final state at 100%:

{
  "keyframes": {"opacity": 0, "transform": "scale(2)"}
}

Shorthand object-form "from-to" format specifies the starting and final states at 0 and 100%:

{
  "keyframes": {
    "opacity": [1, 0],
    "transform": ["scale(1)", "scale(2)"]
  }
}

Shorthand object-form "value-array" format specifies multiple values for starting, final states and multiple (equal-spaced) offsets:

{
  "keyframes": {
    "opacity": [1, 0.1, 0],
    "transform": ["scale(1)", "scale(1.1)", "scale(2)"]
  }
}

The array-form specifies keyframes. Offsets are assigned automatically at 0, 100% and spaced evenly in-between:

{
  "keyframes": [
    {"opacity": 1, "transform": "scale(1)"},
    {"opacity": 0, "transform": "scale(2)"}
  ]
}

The array-form can also include "offset" explicitly:

{
  "keyframes": [
    {"opacity": 1, "transform": "scale(1)"},
    {"offset": 0.1, "opacity": 0.1, "transform": "scale(2)"},
    {"opacity": 0, "transform": "scale(3)"}
  ]
}

The array-form can also include "easing":

{
  "keyframes": [
    {"easing": "ease-out", "opacity": 1, "transform": "scale(1)"},
    {"opacity": 0, "transform": "scale(2)"}
  ]
}

Using the @keyframes CSS rule:

<style amp-custom>
  @keyframes keyframes1 {
    from {
      opacity: 0;
    }
    to {
      opacity: 1;
    }
  }
</style>

<amp-animation layout="nodisplay">
  <script type="application/json">
    {
      "duration": "1s",
      "keyframes": "keyframes1"
    }
  </script>
</amp-animation>

Most CSS @keyframes match the JSON inline keyframes definition in the Web Animations spec with the following nuances:

  • You may need vendor prefixes, such as @-ms-keyframes {} or -moz-transform for broad-platform support. Vendor prefixes are not needed and not allowed in the JSON format, but in CSS they could be necessary.

  • In unsupported platforms, amp-animation's polyfills will fail when using calc() and var() with keyframes specified in CSS. Use fallback values in CSS to avoid this.

  • CSS extensions such as width(), height(), x(), y(), num(), rand(), index(), and length() are not available to @keyframes.

Allowed properties for keyframes

The amp-animation component restricts CSS allowable properties to optimize performance. Below is the allow-listed properties:

Use of vendor prefixed CSS properties is neither needed nor allowed.

Timing properties

Top-level animation and animation components may contain timing properties. Below is the allowed set of properties:

Property Type Default Description
duration time 0 The animation duration. Either a numeric value in milliseconds or a CSS time value, e.g. 2s.
delay time 0 The delay before animation starts executing. Either a numeric value in milliseconds or a CSS time value, e.g. 2s.
endDelay time 0 The delay after the animation completes and before it's actually considered to be complete. Either a numeric value in milliseconds or a CSS time value, e.g. 2s.
iterations number or
"Infinity" or
"infinite"
1 The number of times the animation effect repeats.
iterationStart number/CSS 0 The time offset at which the effect begins animating.
easing string "linear" The timing function used to scale the time to produce easing effects.
direction string "normal" One of "normal", "reverse", "alternate" or "alternate-reverse".
fill string "none" One of "none", "forwards", "backwards", "both", "auto".

All timing properties allow either a direct numeric/string values or CSS values. For instance, 1000 or 1s or 1000m are all valid values for duration.

An example of timing properties in JSON:

{
  ...
  "duration": "1s",
  "delay": 100,
  "easing": "ease-in",
  "fill": "both"
  ...
}

Animation components inherit timing properties specified for the top-level animation.

Variables and calculated expressions

amp-animation allows use of var(), calc(), min(), and max() expressions for timing and keyframes values.

<amp-animation layout="nodisplay">
  <script type="application/json">
    [
      {
        "selector": ".target-class",
        "duration": "4s",
        "delay": "var(--delay)",
        "--y": "var(--other-y, 100px)",
        "keyframes": {"transform": "translate(calc(100vh + 20px), var(--y))"}
      }
    ]
  </script>
</amp-animation>

You may declare CSS variables to use for timing and keyframe values via the var() expressions.

CSS variables are available to nested animations, but nested animations may override the variable's value.

<amp-animation layout="nodisplay">
  <script type="application/json">
    {
      "--delay": "0.5s",
      "--x": "100px",
      "animations": [
        {
          "selector": "#target1",
          "delay": "var(--delay)",
          "--x": "150px",
          "keyframes": {"transform": "translate(var(--x), var(--y, 0px)"}
        },
        ...
      ]
    }
  </script>
</amp-animation>

In the example above:

  • The nested animation applies the var --delay to the delay of #target1 animation.

  • While --x propagates into the nested animation, it is overridden. The ending translate value is 150px.

  • --y is not specified anywhere in the <amp-animation> component. It defaults to 0px if the query does not find it defined as CSS within the <amp style-custom> tags.

Polyfills apply to both var() and calc() on supported platforms. As a best practice, include default values for var().

<amp-animation layout="nodisplay">
  <script type="application/json">
    [
      {
        "selector": ".target-class",
        "duration": "4s",
        "delay": "var(--delay, 100ms)"
      }
    ]
  </script>
</amp-animation>

Override variables of target elements specified in the <style amp-custom> tag by using --var-name fields in amp-animation component. var() expressions first try to resolve values specified within the animation component and then resolve target styles.

CSS extensions

The amp-animation component provides the following CSS extensions: rand(), num(), width(), height(), x(), and y(). The allowed CSS extensions are valid everywhere where CSS values are usable within the amp-animation definition. This includes timing and keyframes values.

CSS index() extension

The index() function returns an index of the current target element in the animation effect. This is most relevant when animating multiple targets with the same effect using selector property. The first target matched by the selector will have index 0, the second will have index 1 and so on.

Among other uses, this property can combine with calc() expressions to create a staggered effect. For instance:

{
  "selector": ".class-x",
  "delay": "calc(200ms * index())"
}
CSS length() extension

The length() function returns the number of target elements in the animation effect. This is most relevant when combined with index():

{
  "selector": ".class-x",
  "delay": "calc(200ms * (length() - index()))"
}
CSS rand() extension

The rand() function returns a random CSS value. There are two forms.

The form without arguments returns the random number between 0 and 1.

{
  "delay": "calc(10s * rand())"
}

The second form has two arguments and returns the random value between these two arguments.

{
  "delay": "rand(5s, 10s)"
}
CSS width(), height(), x() and y() extensions

The width()/height() and x()/y() extensions return the size or coordinates of the animated element or the element specified by the selector. The returned value is in pixels, e.g. 100px.

amp-animation supports the following forms:

  • width(), height(), x(), y() - width/height or coordinates of the animated element.

  • With a selector, such as width('.selector') or x('.selector') - dimension or coordinate of the element specified by the selector. Any CSS selector is usable. For instance, height('#container > li').

  • With a closest selector, such as height(closest('.selector')) or y(closest('.selector')) - dimension or coordinate of the element specified by the closest selector.

The width() and height() are especially useful for transforms. The left, top and similar CSS properties that can use % values to express animations proportional to container size. However, transform property interprets % values differently - as a percent of the selected element. Thus, the width() and height() can be used to express transform animations in terms of container elements and similar.

These functions can be combined with calc(), var() and other CSS expressions. For instance:

{
  "transform": "translateX(calc(width('#container') + 10px))"
}
CSS num() extension

The num() function returns a number representation of a CSS value. For instance:

  • num(11px) yields 11;
  • num(110ms) yields 110;
  • etc.

For instance, the following expression calculates the delay in seconds proportional to the element's width:

{
  "delay": "calc(1s * num(width()) / 100)"
}

Override effects on subtargets

Override timing properties or variables defined in the top-level animation with subtargets. Define subtargets via subtargets: [] where desired, in the same space as valid selectors. Specify a subtarget by index or a CSS selector.

{
  "selector": ".target",
  "delay": 100,
  "--y": "100px",
  "subtargets": [
    {
      "index": 0,
      "delay": 200
    },
    {
      "selector": ":nth-child(2n+1)",
      "--y": "200px"
    }
  ]
}

In the example above:

  • The top-level animation defaults targets matched by ".target" to a delay of 100 and "--y" of 100px.

  • "subtargets": [] includes the first target, "index": 0. This definition overrides the default delay of 100 to 200.

  • "subtargets": [] includes "selector": ":nth-child(2n+1)". This definition overrides the --y variable's default value of 100px to 200px.

Multiple subtargets can match one target selector element.

SVG animations

SVGs are awesome and we recommend their use for animations!

The amp-animation component supports SVG animations with the allowed listed CSS keyframe properties, with the following nuances:

  • IE/Edge SVG elements do not support CSS transform properties. While the transform animation itself is polyfilled, the initial state defined in a stylesheet is not applied. If the initial transformed state is important on IE/Edge, it's recommended to duplicate it via SVG transform attribute.

  • It is impossible to polyfill transform-origin for IE/Edge. For compatibility, use only the default transform-origin.

  • Use CSS transform-box to avoid transform-origin interpretation problems. See issues for Chrome, Safari, and Firefox.

Compatibility and fallbacks

Use media queries, support conditions and switch statements for platform compatibility and fallback options.

Media queries

Specify media queries with the media property. This property can contain any expression allowed for Window.matchMedia API and corresponds to @media CSS rule.

When specified, the animation component will only execute when the environment supports the specified CSS feature.

Supports condition

Specify supports conditions using the supports property. The supports property contains any expression allowed for CSS.supports API and corresponds to @supports CSS rule.

When specified, the animation component will only execute when the environment supports the specified CSS feature.

Animation switch statement

In some cases, you may need to combine conditional animations with an optional default into a single animation. Use the switch animation statement to define the conditions.

{
  // Optional selector, vars, timing
  ...
  "switch": [
    {
      "media": "(min-width: 320px)",
      "keyframes": {...},
    },
    {
      "supports": "offset-distance: 0",
      "keyframes": {...},
    },
    {
      // Optional default: no conditionals
    }
  ]
}

The amp-animation component evaluates switch animation definitions in the defined order. It executes the first animation to match the condition and ignores the rest.

In the example below, the animation runs motion-path animation if supported and falls back to transform:

{
  "selector": "#target1",
  "duration": "1s",
  "switch": [
    {
      "supports": "offset-distance: 0",
      "keyframes": {
        "offsetDistance": [0, "300px"]
      }
    },
    {
      "keyframes": {
        "transform": [0, "300px"]
      }
    }
  ]
}

Combine and split animations

Animations defined in amp-animation can reference each other. This ability allows combining multiple amp-animation declarations into a single animation. Splitting up animations into different amp-animation components allows the reuse of smaller animations, while enabling the same effect as nesting.

<amp-animation id="anim1" layout="nodisplay">
  <script type="application/json">
    {
      "animation": "anim2",
      "duration": 1000,
      "--scale": 2
    }
  </script>
</amp-animation>

<amp-animation id="anim2" layout="nodisplay">
  <script type="application/json">
    {
      "selector": ".target-class",
      "keyframes": {"transform": "scale(var(--scale))"}
    }
  </script>
</amp-animation>

The example animation above combines "anim2" animation as part of "anim1". The "anim2" does not require a selector target. In such case, the included animation references its own target.

Another form allows the including animation to provide the target or multiple targets. In that case, the included animation executes for each matched target.

<amp-animation id="anim1" layout="nodisplay">
  <script type="application/json">
    {
      "selector": ".target-class",
      "animation": "anim2",
      "duration": 1000,
      "--scale": 2
    }
  </script>
</amp-animation>

<amp-animation id="anim2" layout="nodisplay">
  <script type="application/json">
    {
      "keyframes": {"transform": "scale(var(--scale))"}
    }
  </script>
</amp-animation>

In the example above, "anim2" executes for each matched ".target-class". Variables and timing properties specified in the caller animation pass to the combined animation.

Triggering via on action

For instance:

<amp-animation id="anim1" layout="nodisplay">
  ...
</amp-animation>
<button on="tap:anim1.start">Animate</button>

Accessibility considerations for animations

If you are using animations to convey meaning or content, make sure that this is also conveyed in some other form for users who may not be able to see those animations. At the most basic level, make sure that your text content conveys the same information as the animation. For instance, if you're using an <amp-animation> to illustrate a sequence of steps in a process, make sure that there is also text (either on the same page, or in a linked page) that describes the same sequence of steps in words.

Animations can't usually be paused/stopped by users. This can, depending on the type of animation, its size, and whether it loops/repeats or not, be a minor distraction, or a major problem for certain user groups - particularly, if the animation contains fast strobing color changes. In general, we recommend avoiding the use of large, infinitely repeating animations, unless you are certain that they won't have an adverse impact on users. Consider providing a control to allow users to pause an animation. Consider taking advantage of the prefers-reduced-motion media query, and only having an animation take effect if the user has not indicated a preference for reduced motion/animations.

<amp-animation ... media="not (prefers-reduced-motion: reduce)">
  <!-- this animation will only play if the user has *not*
       expressed a preference for reduced motion -->
  ...
</amp-animation>

You can take this further and provide separate, more subtle fallback animations to take effect when prefers-reduced-motion: reduce does evaluate to true, or split out smaller animations that should happen in all cases, regardless of the media feature.

<amp-animation ... media="(prefers-reduced-motion: reduce)">
  <!-- fallback subtle animation effects that only play if the user
       has expressed a preference for reduced motion -->
  ...
</amp-animation>

<amp-animation ...>
  <!-- general/common animation effects that will take effect
       regardless of any user preference for reduced motion -->
  ...
</amp-animation>

See MDN - prefers-reduced-motion and this introductory article on web.dev - prefers-reduced-motion: Sometimes less movement is more for further details.

For animations that are not purely decorative/visual effects, but actually convey information, make sure that any important text and graphical/non-text elements have sufficient color contrast. See web.dev color and contrast accessibility for an introduction (primarily around text contrast) and Knowbility: Exploring WCAG 2.1 — 1.4.11 Non‑text Contrast for more details around non-text elements.

Attributes

trigger

Determines when the animation should be triggered. This must be set to visibility so the animation starts when a story page becomes visible and active.

layout

Should always be nodisplay.

id (optional)

The id of the animation component. Used to reference the animation and chain a sequence of animations.

Actions

start

Starts the animation if it's not running already. Timing properties and variables. Can specify as action arguments. E.g. anim1.start(delay=-100, --scale=2).

restart

Starts the animation or restarts the currently running one. Timing properties and variables. Can specify as action arguments. E.g. anim1.start(delay=-100, --scale=2).

pause

Pauses the currently running animation.

resume

Resumes the currently running animation.

togglePause

Toggles pause/resume actions.

seekTo

Pauses the animation and seeks to the point of time specified by the time argument in milliseconds or percent argument as a percentage point in the timeline.

reverse

Reverses the animation.

finish

Finishes the animation.

cancel

Cancels the animation.

Validation

See amp-animation rules in the AMP validator specification.

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