It’s open source and runs on all kinds of platforms. Original HL1 runs on old Windows and IIRC DOS. Nowhere else
Valve updates HL1 every few years so it runs on contemporary platforms. DOS was ancient history by the time HL came out, you might be getting it mixed up with Quake1
Valve updates HL1 every few years so it runs on contemporary platforms. DOS was ancient history by the time HL came out, you might be getting it mixed up with Quake1