What law is described by the fact that carbon dioxide consists of 27.3% carbon and 72.7% oxygen by mass?

It is the law of definite proportions, which was first stated by Dalton.

The law of definite proportions state that a chemical compound has always the same chemical composition, this is the ratio of the different elements that form it is uniform and the same. In other words, for the example of carbon dioxide, it means that any sample of pure carbon dioxide that exists in any part of the universe has the same ratio of Carbon to Oxygen.