While the federal government generally sets vehicle emission standards, the Environmental Protection Agency has at times granted a waiver to California – because of its longstanding air-quality problems – to set tougher standards. Given the state’s size, automakers have long hewed to its tighter emissions controls even for cars sold in other states.

It’s not clear how much practical impact the court’s decision will have. President Donald Trump seems almost certain to withdraw the waiver, just as he did during his first administration. The waiver was later reinstated by President Joe Biden.