In order for you to play Left 4 Dead on macOS with minimal graphics settings, your computer must meet the following requirements
Operating System:
MacOS X 10.6.4 or higher (Snow Leopard Graphics Update required)
Processor (CPU):
Dual core Intel processor, 2GHz or better
Video Card:
ATI Radeon 2400 or higher / NVIDIA 8600M or higher / Intel HD Graphics 3000
Not supported:
OS X 10.5.x, ATI X1600 or X1900 graphics, NVIDIA GeForce 7 graphics or Intel graphics less than Intel HD 3000.