Exception thrown when calling replaceSync() on non-constructed stylesheet: NotAllowedError Number of CSS rules after replaceSync(): 2 Rule: .test { font-size: 14px; } Rule: .test2 { font-size: 16px; } cssRules returns the same object before and after replaceSync(): true @import rule should be not appear below: Rule: .test { padding: 100px; } Calling replaceSync() while the disallow modification flag set throws: NotAllowedError