Why should a frontend developer learn about Design? Maybe he shouldn't. But he should be ready to fight back when he sees non-realistic and non-performant design proposals. He should be ready to make small decisions when there's no designer around. He should advocate for well-implemented designs when the stakeholders and backend developers say it is "near enough" if the functionality is already there. Understanding and being able to discuss Design doesn't make him a designer, but someone ready to face all these challenges.