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Section 11.2.2 is titled gb18030 encoder, but the first sentence reads, "gb18030's decoder has an associated gbk flag (initially unset)." This is a typo and decoder should be fixed as encoder, as the section is about the gb18030 encoder.
https://github.com/whatwg/encoding/commit/3f4fdded1e2f5f84159d221cfd60148d70ca808f