Pure samples of water always yield 88.89 % of oxygen and 11.11 % of hydrogen, by mass. This can be explained by using the law of:

The law of constant proportions states that a given chemical compound always contains its component elements in a fixed ratio by mass and does not depend on its source and the method of preparation.
Therefore, the ratio of $H:O$ in water = $11.11:88.89=1:8$ which will be constant in any sample of water irrespective of the source and the method of preparation.