I'm with the others on not opening the cap when the engine is hot. Not only does it tell you nothing about the health of the head gasket, it is potentially very dangerous.

The now lower pressure means the boiling point of water is lower and the residual heat of the engine block starts to boil water and the bubbles find their way up to the highest point. And if you're living at a somewhat higher altitude, then the boiling point of water is lower than the 100 degrees C it is at sea level.

Once the engine has cooled down fully, take the cap off the reservoir and start the engine. If straight away bubbles appear and keep coming, then you have a big head gasket leak.
Smaller leaks can be detected by the device already mentioned.

In your video, I couldn't tell if there was oil floating on the coolant, but if things start to look like this in the reservoir, it's not OK either: