In case of overlap the foremost element will hide the former element. The stacks are ordered according to the z-index property. Boxes with "z-index" property form a local stack. Trivia The birth of the web Tim Berners-Lee, a British scientist at CERN, invented the World Wide Web (WWW)...
By the way, CERN is now called the Large Hadron Collider (LHC). When it's used to smash protons into each other we do expect an explosion. But the energy is small compared to the above, and again, you will never get more energy out of the collision than you put ...