StingingVelvet: I had it in offline mode for 6 months straight when I volunteered overseas. No issues. Also I have a bunch of puzzle games installed on the wife's laptop in offline mode. No issues, though I do sign-in there occasionally to switch games.
Lucky you. The plethora of posts on Steam's forums complaining it over a period stretching +6 years or more shows that "but it works for me" hasn't been a universal experience. Same with other related stuff, eg, downloaded Steam mods stop working for some games in Offline Mode.
In any case, the point of this thread is what games on Steam have no DRM at all and still work if you uninstall the client completely / reinstall Windows + copy the game folder over without the client.