You can, but its a pain to get it to work.
I used to play HL2 in school on my laptop, but it would be a 50/50 shot. Steam is ornery about going in offline mode. I would load the game right before I left for school and then sometimes it'd work, then sometimes it'd say it wasn't the latest version or patch or something. Steam is more of a hassle than what its worth when it comes to offline lanning.
Some other things to consider re: Steam and why it sucks...
Each machine must connect to the steam server at lease once or it won't work at all. (HUGE security issue)
Each machine must have it's own steam account
Each machine must have a unique serial number for every steam app installed...
... or a site license [$200 per computer per event (up to 3 days) for HL2, more for CS, CSS, TF2, etc... last time I checked]
All in all, not a good idea
(There's a reason most big CSS, et al. events are bring your own rig)