The WPT test at imported/w3c/web-platform-tests/web-animations/interfaces/KeyframeEffect/iterationComposite.html is failing reliably, flaky or timing out.
<rdar://problem/41000217>
This is expected as we don't support this feature yet.
https://wpt.fyi/results/web-animations/interfaces/KeyframeEffect/iterationComposite.html?label=experimental&label=master&aligned says we pass this.