firefox/testing/web-platform/tests/html/syntax/charset/README.md

These tests don't necessarily match the spec as of September 2021 but instead these tests test the behavior that on one hand WebKit and Blink and on the other hand https://github.com/whatwg/html/issues/6962 have in common.

The cases where the two approaches differ are in https://searchfox.org/mozilla-central/source/testing/web-platform/mozilla/tests/html/syntax/charset