Counter-Strike: Classic Offensive is a remake of Counter-Strike being made inside a remake of Counter-Strike.

Yesterday the modder Z00L released a launch trailer for his curious mod, a project that aims to reproduce the look and feel of the original Counter-Strike (version '1.6' as it's more colloquially known) inside CS:GO. "The main goal of the mod is to get the gameplay from 1.6 right into CS:GO including weapons, sounds, movement, all the old stuff you've dreamed to see in CS:GO," he writes on ModDB. "As you can see, I'm pretty near."

The mod is built within CS:GO's version of Source, and it'll require CS:GO to play. At launch, planned December 25, Z00L says that retro versions of Dust2, Italy, Mirage and Inferno will be playable. Each of these maps exist in the current version of CS:GO, of course, but they've since been aesthetically and structurally reimagined in small or significant ways. 

As stated in August, Z00L's goals with the project are to make weapons that behave similarly to 1.6, remove 'GO'-specific guns, replace all sounds, and remove skins. He also outlines what he is not able to do as a result of the engine:

• Wallbangs as in 1.6 (even if I change it it won't penetrate more than 32units)
• Remake the crouching jumping bug (no I won't edit the dlls)
• Playermodel selection (a plugin could fix that though)
• See inside smoke particles (the game don't draw the world when inside, so nope..)
• Bring back the tactical shield
• Change flashes, smoke time, nade physics
• Add buying ammo

So although the project is appetizing to folks like me who grew up playing 1.6 in internet cafes, it does seem to be operating under some fundamental constraints that might make it impossible to include certain movement quirks and 'desirable' map bugs what were buffed out over Counter-Strike's different iterations. It's hard to tell from the in-game trailer exactly how well Classic Offensive captures the movement and weapon feel of old CS, but to my eyes it resembles the higher-fi Counter-Strike: Source more than anything. I guess that isn't unsurprising, considering it's the link between 1.6 and GO.

Which version of Counter-Strike was the best, the most pure, or the most tactically interesting remains a hotly debated topic by FPS players. For the year following its release in 2012, CS:GO wasn’t even the most popular version of Counter-Strike—some players were still actively arguing the merits of GO against its thirteen- and nine-year-old predecessors.