Few people have noticed it, but Obama pulled a rather clumsy legalistic trick when he talked about the "norm" that chemical arms should not be used.
The 1925 Geneva Protocol forbids the use of chemical arms in wars between states. However, it does not forbid their use in civil wars. Neither does it forbid their use once the other side has used them.
This became a problem in 1988 when Saddam attacked Halabja with chemical arms. In a reaction a new and much stricter treaty, the Chemical Weapons Convention, was adopted in 1993. But while Assad signed the Geneva Protocol he never signed the convention.
So Obama had a problem with his red line as Assad didn't violate any treaty if he had used chemical arms. One because it doesn't forbid using them in a civil war, the other because he hadn't signed it. Of course Obama could have resorted to more general treaties about human rights and protection of civilians in a war, but that would make his red line statement specifically about chemical arms look a bit weird.
Obama's solution was to claim that so many countries have adopted the Convention that it has become the international "norm".
It is a rather ugly construction. It raises also the question whether it can be applied more broadly. The US has a bit of a habit to be the only country that doesn't sign some treaties. Should we hold them to those treaties anyway because they become the "norm"?