Expectations presumed the pref for it (`layout.css.import-supports.enabled`) was always on and didn't catch in testing as it is enabled in nightlies, fixed in patch just sent.
Bug 1830050 Comment 2 Edit History
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Expectations presumed the pref for it (`layout.css.import-supports.enabled`) was always on and didn't catch in testing as it is enabled in nightlies, fixed expectations to account for the pref value in patch above.