do I have to type the alt=''' ?

Answer 1

Yes! Your site isn't going to break if you don't put it in there, but in theory it would be invalid HTML. Get used to adding it, it's not a big deal. And while you're at it, add some useful text to it and don't just leave it empty :P

Answer 2

Of course! It is REQUIRED

Answer 3

not necessary try it by removing it?