the HL2 engine CAN use normalmaps its just that it doesnt uses everywhere (in HL2) or even nowhere (even if it still can,and you can make a mod to it using normalmaps) in CSS. But thats a designer choice not a limitation of the engine (and you notice it in frames per second: comparing DOOM3 where almost anything has a normalmap and goes at 34fps in my machine to HL2 that only uses it where it matters most and i get 78fps...)