Single Player is the only mode which can be played with Steam set to offline. Other than that, the same rules as Campaign apply here, except for in Dead Center, or The Passing where you only have to get 8 Gas cans in Solo Dead Center (13 in standard Campaign mode) and 10 in Solo The Passing (16 in standard Campaign mode).

No other player can join a Single Player game, unless locally (LAN), via the connect 'host local ip' console command. The player assumes control of one of the Survivors, and all the other characters are AI controlled. Single Player is a gameplay mode with only one player controlled character. Available in Left 4 Dead and Left 4 Dead 2 (Pre- The Last Stand Community Update) Play a single player offline game with computer-controlled teammates.