Can be read as "U.S. imposes its first-ever national drinking water limits" or "U.S. imposes world's first-ever national drinking water limits". There's a missing word and it's not clear from context what it's meant to be.
Using emphasis doesn't help bring clarity since the problem is the headline has multiple valid interpretations regardless if said words are emphasized or not. Adding something like "imposes its" helps highlight the intended interpretation.