What data does a sample of pure water irrespective of its source contains 11.1 hydrogen and 88.9 oxygen support?

It might have supported Proust's law or the law of definite composition, or law of constant composition. But unfortunately the data used are not accurate. The correct proportions, based on relative atomic masses of hydrogen and oxygen being 1.008 and 15.999 give the results as 11.2% and 88.8%.