Just did this with my HP Envy injket and it worked, had no idea this was a thing it's very cool.

Yeah, I'm surprised this works. I expected some complex printer-specific reverse engineered protocol.

The modern "protocol" is PDF. That's what everyone standardised on during the rise of smartphones and needing to have them able to print.

PDF includes pretty much all the printer needs, like pagesize, colour palette, font information, etc.

It's not the greatest format in the world... But everyone can shove in one of dozens of PDF parsing libraries without a lot of effort. So it happened.

The document format is PDF (or postscript before that), the protocol is IPP.