Didn't you have a Voodoo-card or something? I'm quite sure I could run HL1 on a P166 with a Voodoo-card with ok frame rate.
Still remember the concern when Quake 3 would require a GPU as they were not entirely ubiquitous then. It was the right more and it helped the industry forward but it was a little pain point at that time.
Quake 2 and Half life had pure software renderer for people who didn't have 3D acceleration cards such as Voodoo.
It was slow but worked on ANY graphical card :)