Can I practise singing without a piano or a teacher?
Yes, for the parts that need a reference pitch and a routine — scales, sustained notes, interval training and warm-ups all work from a played reference, and a browser can play one as well as a piano can. What you lose without a teacher is diagnosis: nobody is telling you that the tension you cannot hear is in your jaw. A reasonable compromise is to work daily on your own and take an occasional lesson to check nothing has gone wrong.