Unless the feds can somehow challenge California's authority, the state may hold both the White House and car companies over a barrel. The Clean Air Act gives California a waiver to set its own emissions standards, and 12 other states (including New York and Pennsylvania) follow those standards. If a conventional car maker wants a reasonable chance of success in the US, it has to honor California's guidance -- effectively, one state is frequently determining the rules for everyone else.

The approval is likely to please those concerned that automotive standards would slide backward under a federal leadership determined to protect the fossil fuel industry. However, it's definitely not going to make car companies happy, and might create issues if you're a driver. The Obama administration's speed in locking in standards pre-Trump caught automakers off-guard -- they think officials gave them too little time to think about the impact. And the new standards are estimated to raise the price of a car by about $1,000 in 2025, which isn't trivial for most buyers.