One thing is testing if the fancy custom react component works properly in a particular browser at all, that should be the case in a certain browser+version across OSes and you'd want that available all the time.
The other thing is testing if some CSS is acting up in a specific Browser/Browserversion/OS/Screensize/Phone combination; that is probably a job for QA.
The other thing is testing if some CSS is acting up in a specific Browser/Browserversion/OS/Screensize/Phone combination; that is probably a job for QA.