Calm down. This is not real. But it is a nicely done video that imagines a Super Smash Bros. Melee-type game with a roster full of fan favorites from Valve's stable of characters.
The casts of Left 4 Dead and Left 4 Dead 2 and Team Fortress 2 join Chell, Gordon Freeman and all the rest in this mash-up. As best as he could, YouTuber crazyhalo tried to match this trailer shot-for-shot with the original Super Smash Bros. Melee trailer. Not all characters have the same actions or movements but where possible, the same colors, lighting or similar backgrounds are used.
You can see the side-by-side comparison below.