🚨 Error: Failed to execute 'animate' on 'Element': Partial keyframes are not supported.

Details
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**Error report:** [link](http://go/ampe/COXNyPza9eWBnAE)
**First seen:** Jan 8, 2022
**Frequency:** ~ 243/day

Stacktrace
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Error: Failed to execute 'animate' on 'Element': Partial keyframes are not supported.

    at animate (extensions/amp-animation/0.1/runners/native-web-animation-runner.js:72:36)

    at map (extensions/amp-animation/0.1/runners/native-web-animation-runner.js:67:35)

    at init (extensions/amp-animation/0.1/runners/native-web-animation-runner.js:97:11)

    at start (extensions/amp-story/1.0/animation.js:358:11)

    at startWhenReady_ (extensions/amp-story/1.0/animation.js:478:22)

Notes
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`@rsimha` modified `extensions/amp-animation/0.1/runners/native-web-animation-runner.js:72-73` in [#23417](https://github.com/ampproject/amphtml/pull/23417) (Jul 19, 2019)
`@renovate-bot` modified `extensions/amp-animation/0.1/runners/native-web-animation-runner.js:67-67` in [#27350](https://github.com/ampproject/amphtml/pull/27350) (Mar 30, 2020)
`@aghassemi` modified `extensions/amp-animation/0.1/runners/native-web-animation-runner.js:81-107` in [#20831](https://github.com/ampproject/amphtml/pull/20831) (Feb 18, 2019)
`@alanorozco` modified `extensions/amp-story/1.0/animation.js:358-358` in [#12453](https://github.com/ampproject/amphtml/pull/12453) (Dec 18, 2017)
`@rsimha` modified `extensions/amp-story/1.0/animation.js:477-480` in [#21212](https://github.com/ampproject/amphtml/pull/21212) (May 16, 2019)

**Seen in:**
- 01-07 Nightly (1715)

**Possible assignees:** `@rsimha`, `@aghassemi`

/cc @ampproject/release-on-duty