When it comes to combining certain things, there are people in this world who say "why not", instead of "why". Like the person who played Hotline Miami and thought, "Gee, wouldn't Half-Life be cool like that."
That's essentially what we're dealing with here: take one of the most iconic FPS games ever made, and now run through the entirety of the campaign with a top-down view. It's almost like a horror game at points: the corridors and elevators have such low ceilings, which forces the camera to zoom into exceedingly uncomfortable angles. Not great for fending off leaping head crabs.
Because you obviously can't aim up or down in this mode, creator Sockman111 has beefed up the auto-aim. Even still, they're not sure if you can play the whole game like this, but it's at least technically possible, which is neat in and of itself.
You can see three minutes of gameplay from the opening levels below. The full mod can be found on ModDB here, if you love the aesthetic enough to put yourself through this kind of torture.
I wonder how far the camera zooms out in the Xen levels.