What gas is contained in the bubbles of boiling water?

http://scienceline.ucsb.edu/getkey.php?key=3197


It is water. We think of it as a liquid but water can exist as a gas
and a small amount does even at room temperature. A standing body of water (or any liquid) continuously loses small amounts of its molecules to the air as they get excited to the gaseous state (evaporation). The concept of vapor pressure describes the tendency for molecules to escape a liquid. The counter force that keeps a lake from vanishing overnight, is atmospheric pressure which pushes down and keeps these renegade escapee liquid molecules in check.
As a liquid is heated, the vapor pressure rises, to the point where the
vapor pressure from within the liquid equals the atmospheric pressure...as more heat is applied beyond that point the balance is shifted in favor of the vapor and a "prison break" occurs: boiling.

It should be noted as answer one in this link describes, that the tiny bubbles you see early on while heating up to boiling, are dissolved
air falling out of the water

As a consequence of the first explanation, the more air pressure, the more heat needs to be applied for the vapor pressure to overcome that air pressure. Therefore water boils at a lower temperature at high altitudes (with lower air pressure) and at a higher temperature in a pressure cooker, allowing food to cook faster.