No.

Dangerous for the machine yes. It can overheat

and eitehr shut it self off or get damaged if it fails to shut down

It is moderately dangerous to your laptop. A laptop should be placed on a hard, dust free surface. Sitting on your stomach, assuming with clothes on, the fan will pick up fibers and dust from your placement. The fan will clog sooner, causing overheating eventually.

dangerous? if your laptop is like most, it'll power down automatically a few minutes after you close it. then it's no-longer dangerous to either it or you

In order to receive certification and be licensed to manufacture and distribute a product, a company must prove their product us safe to use, and apply specific warning labels if there is any danger presented from impropet use. A laptop cannot be dangerous (unless its an undetected flaw introduced during the manufacturing process), or it never would have been sold in the first place

Dangerous in what way...? It shouldn't produce enough heat exhaust to burn you, and the radiation such a device would emit would be so minimal that in a thousand years it wouldn't effect you. Ideally, you want the fans on the bottom of the laptop to have enough clearance to allow proper suction, which I suppose could be hindered by your shirt potentially blocking the vents... So, at worst, you're affecting your laptop's cooling system, but you'd know quick if it was affected.