It's normal that, with a fresh bottle created by CrossOver Games 8, there would be no Direct3D key under [HKEY_CURRENT_USER\Software\Wine]. You can create it as necessary.
Second, with CrossOver Games 8, OffscreenRenderingMode defaults to "fbo". So, you don't have to set that. You might experiment with setting it to "backbuffer" to resolve specific problems, but it's almost always better to have it set to "fbo".
It's a good idea, although typically not necessary, to set VideoMemorySize to the number of MB of VRAM your graphics card has. Although you're entering a number, you should create a string value in regedit. For example, it might be 256 or 512 -- it's just the number, no units or label (i.e. don't put "256 MB"). CrossOver tries its best to guess the amount of VRAM on your card based on its model, but it might guess wrong which might crimp performance.
If you're having problems with a game, you should experiment with setting useGLSL to both "enabled" or "disabled". CrossOver Games 8 has greatly increased capability for the GLSL-disabled case. In fact, disabling GLSL may give better results than enabling it, for some games, GPUs, or configurations. So, it's worth experimenting. If useGLSL is unspecified, CrossOver Games 8 uses a heuristic based on the capabilities of your GPU to decide whether to use GLSL or not.
All of these settings may go in either [HKEY_CURRENT_USER\Software\Wine\Direct3D] or [HKEY_CURRENT_USER\Software\Wine\AppDefaults\left4dead.exe\Direct3D]. (Again, you would need to create any keys which don't already exist. Also, you can use this for other programs by using their .exe file name instead of "left4dead.exe".) The former affects all games, the latter only affects Left4Dead. (VideoMemorySize is appropriate for the global key, the other values may be better as Left4Dead-specific.) By putting potentially-dangerous settings in the app-specific Direct3D key, you avoid the need stated in the Tips&Tricks for using a separate bottle.