I understand both sides of the argument, so I set it up so you could choose which one you preferred. I don't see a reason to force my preferences on anyone.
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"
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.
The only real practical concern is that if you don't use data- you might use an attribute that will become meaningful to future browsers. But what are the chances of html gaining an attribute that starts with hx-?
When you camp on the platform namespace it ties the hands of standards bodies - we will not get <modal> in HTML for real because too many people already went ahead and did it. Same for <accordion>, etc. Look up 'Smooshgate' for a similar example in JavaScript.
Our workflows should have us practice writing _valid_ code, which our tools can then take and transform into other _valid_ code. Any workflow where the humans practice doing the _invalid_ thing so the tool can magically fix it later is a bad idea IMO. Practice writing good code and use tools to make it even better!
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.