Run the game while online to ensure all files are verified. Go Offline: In the Steam menu, select "Go Offline."
The built-in bot system allows for customizable difficulty levels, making it ideal for casual solo play. How to Get Counter-Strike: Source for Offline Play
If you want to set up a specific offline environment for a local network (LAN), you can use the SteamCMD tool to download the "Dedicated Server" files. This allows you to host a match on one machine that others can join via Ethernet, even if the internet is down. Top Features to Look for in a CSS Offline Build