First off, apologies this might sound trivial to some people, but I quite like (some) video games and DRM gets right up my nose. Currently you're unable to put Steam into offline mode to play the games you've already downloaded without first connecting to the Internet, and that isn't right.
I've currently found myself between homes and do not have a stable Internet connection. I can get some limited access via my mobile phone but some ports are blocked (including the ones that steam uses).
I anticipated this and put Steam into offline mode using these instructions (https://support.steampowered.com/kb_article.php?ref=3160-agcb-2555)
before my connection to the Internet was cut. Fine. Everything worked in offline mode for about two weeks (because I was previously connected to the Internet).
When offline mode stopped working and Steam would complain that it was "Unable to connect to the Steam network..." I sent Steam a support ticket asking "How do I get steam into offline mode without first connecting to the Internet?"
Their solution was to reinstall Steam. Obviously that didn't work. Then I was asked to send them a MSINFO32 report which would obviously be no use so I took my PC to a friends house instead and connected to his Internet, connected to Steam, put steam back into offline mode, and came home again.
Crucially I took a backup of the Steam files after I put it into offline mode for the second time.
After about another 2 weeks of using Steam in offline mode the same thing happened again. But this time I was on the case. But after restoring the backup it still didn't work, so I thought I'd try changing the date on my PC. I flicked the date down a couple of days and et voilà! Offline mode is back up.
So there is some sort of time limited DRM on steam offline mode, and it will stop working after about 2 weeks. You have been warned. DRM sucks. Etc.