It looks like that code was introduced here: https://github.com/bigskysoftware/htmx/commit/5da5dc60639370b8174cdf998ebfc7153430cea1#diff-f9273837c3a0dc0b7e021b5e513963904f99fdf23e60d3f4e2e5ad3b6906b5b0R1060 That pre-dates those headers, I think