Not only do browser manufacturers disregard that request, I observe that IETF has gone back on their idea of prefixing application protocols with 'X-`. See RFC 6648: "Deprecating the "X-" Prefix and Similar Constructs in Application Protocols"

https://tools.ietf.org/html/rfc6648

If the HTML standard changes so much as to collide with 'hx-` then application developers using HTMX will likely have bigger compatibility problems to deal with. Besides, regex replacement on your HTML templates isn't hard.