You want the error to still be there if the user focuses on the input again, though. And you also want to error to appear immediately when the user blurs the input - you don't want them to be three fields ahead when you finally tell them about their first error!

In general I agree that we should be looking for CSS/built-in solutions to these sorts of input state problems, but this is one of those cases where you either use JavaScript, or accept the accessibility hit. (Bearing in mind that JavaScript itself can be an accessibility hit in its own right.)