🚨 Error: Failed to read the 'cssRules' property from 'CSSStyleSheet': Cannot access rules

Details
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**Error report:** [link](http://go/ampe/CJT006TE2fCvPQ)
**First seen:** Sep 29, 2021
**Frequency:** ~ 14,659/day

Stacktrace
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Error: Failed to read the 'cssRules' property from 'CSSStyleSheet': Cannot access rules

    at cssRules (extensions/amp-animation/0.1/parsers/keyframes-extractor.js:38:18)

    at scanStyle (extensions/amp-animation/0.1/parsers/keyframes-extractor.js:18:22)

    at this (extensions/amp-animation/0.1/web-animations.js:467:41)

    at createKeyframes_ (extensions/amp-animation/0.1/web-animations.js:447:29)

    at callback (extensions/amp-animation/0.1/web-animations.js:604:10)

    at callback (extensions/amp-animation/0.1/web-animations.js:1004:19)

    at withVars (extensions/amp-animation/0.1/web-animations.js:602:18)

    at callback (extensions/amp-animation/0.1/web-animations.js:988:19)

    at withTarget (extensions/amp-animation/0.1/web-animations.js:597:16)

Notes
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`@dvoytenko` modified `extensions/amp-animation/0.1/parsers/keyframes-extractor.js:35-46` in [#9623](https://github.com/ampproject/amphtml/pull/9623) (May 31, 2017)
`@dvoytenko` modified `extensions/amp-animation/0.1/parsers/keyframes-extractor.js:16-18` in [#9623](https://github.com/ampproject/amphtml/pull/9623) (May 31, 2017)
`@alanorozco` modified `extensions/amp-animation/0.1/web-animations.js:467-467` in [#27887](https://github.com/ampproject/amphtml/pull/27887) (Apr 21, 2020)
`@dvoytenko` modified `extensions/amp-animation/0.1/web-animations.js:445-453` in [#9614](https://github.com/ampproject/amphtml/pull/9614) (May 31, 2017)
`@dvoytenko` modified `extensions/amp-animation/0.1/web-animations.js:604-610` in [#9614](https://github.com/ampproject/amphtml/pull/9614) (May 31, 2017)
`@dvoytenko` modified `extensions/amp-animation/0.1/web-animations.js:995-1009` in [#9614](https://github.com/ampproject/amphtml/pull/9614) (May 31, 2017)
`@dvoytenko` modified `extensions/amp-animation/0.1/web-animations.js:602-602` in [#9614](https://github.com/ampproject/amphtml/pull/9614) (May 31, 2017)
`@dvoytenko` modified `extensions/amp-animation/0.1/web-animations.js:988-989` in [#9496](https://github.com/ampproject/amphtml/pull/9496) (May 24, 2017)
`@dvoytenko` modified `extensions/amp-animation/0.1/web-animations.js:596-597` in [#10451](https://github.com/ampproject/amphtml/pull/10451) (Jul 18, 2017)

**Seen in:**
- 01-07 Beta (1715)
- 01-07 Control (1715)
- 01-07 Experiment-B (1715)
- 01-07 Experimental (1715)
- 01-07 Stable (1715)
- 01-14 Nightly (1909)

**Possible assignees:** `@alanorozco`, `@dvoytenko`

/cc @ampproject/release-on-duty

Start by reading extensions/amp-animation/0.1/parsers/keyframes-extractor.js at cssRules and scanStyle, then trace their callers in extensions/amp-animation/0.1/web-animations.js at createKeyframes_. Investigate the reported CSSStyleSheet access failure in the affected browser conditions. Done means the production error no longer occurs and the relevant behavior remains covered by the existing project checks.

Written by the indexing model from the issue text.