Direct3D is a graphics application programming interface (API) for Microsoft Windows. It is part of DirectX.
GoldSrc engines ships with Software renderer, OpenGL and initially, Direct3D 7, prior to it being removed in 2013 after SteamPipe update. Unlike OpenGL (since detail textures support was added in 2004 with the release of CS:CZ) and like the Software renderer, it doesn't seems to have issues with overbright lighting.
Source mainly used Direct3D 9 (Shader Model 2) renderer, which later was upgraded to support Shader Model 3.0 (in all games since ), but offers backward compatibility with older GPUs using DirectX feature levels, some third-party engine branch like Titanfall engine branch and Strata Source used Direct3D 11 renderer instead. Later throughout 2020-2022, nearly all of Valve games also added support for Vulkan renderer on Windows, using DXVK to translates D3D9 to Vulkan. Enabling Vulkan translation will require the "
-vulkan" launch option (except in Portal with RTX and upcoming Half-Life 2 RTX).
Source 2 mainly used Direct3D 11 renderer, but also have a options to use the Vulkan renderer.