Test:

Reference:

https://searchfox.org/mozilla-central/source/testing/web-platform/tests/css/CSS2/syntax/at-charset-001.xht
https://searchfox.org/mozilla-central/source/testing/web-platform/tests/css/CSS2/syntax/at-charset-001-ref.xht
https://searchfox.org/mozilla-central/source/testing/web-platform/tests/css/CSS2/syntax/support/at-charset-001.css

It's not clear to me how this can happen, but it does.