Every time I've heard a native speaker say this, he or she says "c'est pas grave" instead of ce n'est pas grave. I used c'est pas grave in a Duolingo exercise and was marked wrong.
Is this considered grammatically correct or is it just a colloquial thing that a lot of people say, like ouai, ché pas, j'ai kiffé ça etc ? Duolingo doesn't like j'ai kiffé either.
To what extent is this sort of thing used in casual conversation. Is it an informal register only or is it more general?
The extended French from English course is a lot of work, but I'm getting there. I really like the Italian from French course too. It's much better than Italian from English.