From what I've read online before the state with the strictest vehicle emission laws is California who are pretty darn strict when it comes to the emissions allowed by your vehicle during emission testing.
After all California has warnings on just about everything you buy stating that the items contains chemicals known to the state of California to cause birth defects, cancer and other reproductive harm.
So I would agree that California has to be the strictest on emissions laws and vehicle emissions testing than any other state.
Still several other states are pretty strict when it comes to emissions testing of vehicles before registering or renewing your vehicles registration.
Those states with strict emissions testing laws are Colorado, Connecticut, Delaware, Maine, Maryland, Massachusetts, New Jersey, New Mexico.
Besides those states the other states that are also pretty strict when it comes to emissions testing for vehicles are New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington, as well as the District of Columbia.