Whilst http-equiv is said to be an enumerated value, so far as I know in Chrome, it actually is "magic HTTP Header pixie dust": [0] And whilst in Firefox parsi

Whilst http-equiv is said to be an enumerated value, so far as I know in Chrome, it actually is "magic HTTP Header pixie dust": [0] And whilst in Firefox parsing of the element is more spread out, they accept a very wide number of headers that aren't documented, and you'll find examples on MDN for any number of headers using http-equiv which aren't specified in the standard or on their docs for http-equiv itself [2] (For example, X-DNS-Prefetch-Control [1]). [0] https://source.chromium.org/chromium/chromium/src/+/master:out/Debug/gen/third_party/blink/renderer/bindings/core/v8/v8_html_meta_element.cc;l=95;drc=65edd51497213d077fa26bb4f24ab2341b243074;bpv=1;bpt=1?q=metaelement.c&ss=chromium https://source.chromium.org/chromium/chromium/src/+/master:o... [1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-DNS-Prefetch-Control https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-... [2] https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta#attr-http-equiv https://developer.mozilla.org/en-US/docs/Web/HTML/Element/me...