WebStorm/PhpStorm warning for react attributes in jsx like className
My problem with warnings in jsx was solved by instaling React-Template plugin.
You should add Reactjs library.
Using Reactjs in JavaScript and TypeScript
In my case, I've just added Reactjs support for TypeScript:
I'm a little late on the scene but performing Invalidate Caches/Restart... solved the issue (after switching to JSX Harmony).