App doesn't crash even if I don't mention this line at the top of every layout and drawable file: in Android?

I am not getting a clear explanation which can provide me with substantial (and official) proof as to answer the question, why doesn't my app crash when I don't write

<?xml version="1.0" encoding="utf-8"?>

at the top of every layout and drawable file in Android and if this is not purely necessary or a compulsion that should always be followed then what is the best practice actually: to mention it at the top of every layout and drawable file in Android or to omit it? And if we omit it then how does it actually work as there seem to be no side effects if we don't write it? And if this is the case then why are there no side effects of omitting it?

A logical and detailed answer would be much appreciated.

from App doesn't crash even if I don't mention this line at the top of every layout and drawable file: in Android?