What is Proust's law of constant composition?

A particular compound will always contain same in same (constant) proportion, irrespective of its source or method of preparation

Nowadays, Proust’s Law of Constant Composition is usually known simply as the Law of Constant Composition.

For example, oxygen makes up 88.810 % of the mass of any sample of pure water, while hydrogen makes up the remaining 11.190 % of the mass.

You can get water by melting ice or snow, by condensing steam, from river, sea, pond, etc. It can be from different places: USA, UK, Australia, or anywhere. It can be made by chemical reactions like burning hydrogen in oxygen or by of acid and base (NaOH + HCl).

If the water is pure, whatever its source or method of preparation, it will always consist of 88.810 % oxygen by mass and 11.190 % hydrogen by mass.