If I write my employer an email that says "do x, y, and z or I resign", that is pretty obviously a resignation if x,y, and/or z are refused.

In fact, there is no law that states resignation must be submitted in writing (though a contract can stipulate that requirement). I can walk into my boss' office and resign verbally with immediate effect. California is an "at will" state.

That said, I would be happy to read the source you're citing with respect to the employment law relating to resignation if it contradicts my claims.