[THEORY] All the Valve games are in THE SAME UNIVERSE!
I have a theory that all the Valve games are actually in the same universe. I have facts to support it too! Here is how the timeline would go:
The first game in the timeline is Day of Defeat and Day of Defeat Source. These games take place during World War II, so they would take place some time between 1939 and 1945.
Next is Team Fortress Classic. The TF2 comic, "A Cold Day in Hell" shows that Team Fortress Classic is canon, and takes place within the same universe as TF2. My guess is that TFC would take place 5 or 10 years before Team Fortress 2. This would set it around 1940's or 50's.
Next comes Team Fortress 2. Calenders in the game, and dates in media and comics, state that it takes place around 1960's or 1970's.
The events in Portal, Half Life, Opposing Fource, and Blue Shift probablly take place around the same time. We know that the Black Mesa incident takes place within 5 years before or after Portal. Portal takes place around 1999 or 2000, so the first Half Life games would be around late 1980s or early 2000's.
Next comes Counter Strike games. These games obviously take place before the Combine Invasion, or before the Black Mesa Incident has any effect on the rest of the world. Based on technology and the locations this game should take place somewhere around 2000.
Next comes Half Life 2 and it's episodes. Valve stated that Half Life 2 takes place 2 decades after the first Half Life, so that would set Half Life 2 around the 2020's
Portal 2 would take place after the Combine has been defeated from it's takeover of Earth, because in the ending of Portal 2, when you portal onto the moon and you can see Earth, the oceans are not drained of water, and there seems to be no Combine technology, thus implying that the Combine no longer have control over Earth. Based off of the amount of deterioration to the Enrichment Center, and the expiration dates on the Longterm Relaxation Vault containers, Portal 2 would take place 20 or 30 years after Half Life 2, so it would be 2040 or 2050.
Now comes the Left 4 Dead series. In the games, a zombie apocalypse has occured. There is no evidense of a "zombie apoclypse" in the Portal or Half Life games, and the game could not take place before Portal, because of the time period. So this sets Left 4 Dead sometime during or possibly after Portal 2.
After that is Alien Swarm. Valve offically said that this game takes place in December 2052.
Finallly is Dota and Dota 2. This one should take place EXTREMELY far into the future on Earth, or on some other planet.
At some point the Space Core crashes on the planet Skyrim takes place on. I'm not sure is this can be considered canon, but if it is all I know is that this event takes place some time after Portal 2.
I hope you enjoyed my theory. I think it makes sense, but it really depends on the opinion. Thanks for reading!